Remco.org

Seductive serendipity / Verleidende serendipiteit

April 26th, 2016

FYMAS, MAS QRP contest

A priori: click on images to enlarge in new tabs.

Introduction.
This month I arrived at the QRPcc website by accident.
FYM and QRP?? Well, QRP is not my main thing but this QRP club
organizes contests on Ascension Day since 2000.

Charm and challenge of this contest is to use self built QRP equipment with
minimal components. The less components you have, the more points per QSO you get.

In other words, less is more ; -)

Because I attend the Jutberg, enjoy home brewing and contesting this
MAS contest seems a nice exercise at the end of this years Ascension Day.

Plan is to participate in ‘Class B’, only 40m with a FCP vertical,
using N1MM with my Arduino keyer.

Note: nowhere is stated that ‘QRP’ has to be done with a straight key!

Equipment.
Here you can see some pictures and circuit diagrams of equipment from
participants over the last years.

The MAS contest seems ‘hauptsächlich eine Deutsche Sache’.

So, my aim is to change this a little bit ; -)

Googling on various circuit diagrams for QRP equipment I felt that my entry
had to deliver something new. So . . . I started drawing some circuit diagrams
of minimalistic 40m transmitters -made of junk parts- in my mind.

I excluded tubes, so my 40m transmitter had to be made of transistors and/or FETs.

Circuit diagram.
A Hartley oscillator is one of the most minimalistic designs, it reduces to two components.
I built a Hartley with a small FET. It works, but the frequency stability is horrible.
Peter PA3EXL was so kind to lend me a 7030 kHz crystal, which I used to discipline the Hartley.

My initial idea was to build a Hartley power oscillator with an IRF510 or something.
However, these FETs need biasing to oscillate, which means extra components.
I considered a Pierce oscillator as alternative. Also in this case the IRF needs biasing.

So, I went back to my original idea: crystal disciplined Hartley with amplifier.

After some fiddling, trying to minimize the amount of components, the following
circuit diagram crystallized, named FYMAS (see figure 1 below).

Figure 1. FYMAS, 40m QRP transmitter for 2016 MAS contest.

The oscillator coil (2) is wound around a wooden 28mm diam rod. A hole is drilled in the
centre of this rod to accomodate an adjustable ferrite rod to tweak the frequency a little.
According to the MAS rules such ferrite rod is part of the coil, so it eliminates a capacitor : -)

Unloaded output is a beautiful sine wave with 4Vpp, measured on my oscilloscope.

Unfortunately I got bad results interfacing the oscillator electrically to the gate of the IRF510 (7) .
It was my intention to ‘auto bias’ the FET. It worked a little but revealed low output.

Therefore two additional components (5 and 6) were inevitable to achieve around
2W output @13.8V after some tweaking with 4 and 6.
The IRF510 simply doesn’t receive enough drive to switch ‘firmly’.

More output is possible by raising the power voltage, but I’ll bring only one power supply
at the Jutberg. So, considering the circumstances 2W has to be enough.

Matching circuit.
MAS rules (2016) state (quote):

“Any selective network in the TX output stage will be assumed and counted as a
3 parts PI filter. For a better suppression of harmonics you are free to use
more components – they will not be counted.”

The key word here is ‘any’. Read on . . .

Below my matching circuit is depicted.
Match from RL = 15 + j0 Ω (assuming ca. 5W output) to 50 + j0 Ω @ 7 MHz.

Of course a good match can be obtained by halving the ‘last’ parallel capacitor.
Interpreting the MAS contest rules I deliberately chose NOT to do this.
The last capacitor is intentionally too big. By adding a parallel coil (ca 1.2 uH)
you go ‘back’ in the Smith chart, making this coil part of my selective network.

‘Cold’ side of the coil is connected to +13.8V, requiring a capacitor to make this side low Z.
The capacitor is not included in the above picture but . . .  is a mandatory part of the matching circuit !

So . . . my parts to power the IRF510 don’t count! If they will, the MAS rules have to be changed.

Note: the consequence of my reasoning is also losing a DC blocking capacitor at the output.
This capacitor is not part of a ‘selective network’. Because I will use an ‘open’ antenna, this isn’t an issue.

FYMAS keying.
Initially I wanted to key the transmitter by shortening the oscillator coil tap to ground
to (try to) prevent chirps.

This works with a screw driver, in a sense that the Hartley stops oscillating.
However, it seemed that the IRF510 (7) (see fig. 1 above) in my design was VERY willing to
oscillate around 1 MHz and on its turn was disciplined by the Hartley ; -)

Connecting the (open collector) key output of my keyer to the coil tap stopped the Hartley,
no matter if I keyed or not. Experiments using an ‘intermediate’ BS170 also failed.

Another method of keying the transmitter is to shortcut the IRF510 gate to ground.
This works, but doesn’t stop the Hartley from oscillating, which is unwanted during reception.

(Tip! For those who want to build a transceiver according to my design can benefit from
this by using the Hartley as local oscillator for e.g. a direct conversion receiver)

Also, keying has to be done in an inverted sense, i.e. ‘key open’ = TX.
Luckily my Arduino keyer owns an open collector inverted keying output : -)

Disconnecting the GND side of the coil (2, see fig. 1 above) with a relay (of course) stops the Hartley.
The other part of the DPDT relay is used as RX/TX antenna switch.
Btw, relays do NOT count as parts in the MAS contest.

How does it sound?
Pretty well, showing only a small ‘start up glitch’.

A recording of my signal received ca. 5km away can be heard in MP3 or OGG. Not bad eh ?

Finally, below a picture of the FYMAS, as used in MAS 2016, is depicted. (click to enlarge)

 

April 11th, 2016

Junkbox 10 and 27 MHz GPSDO for Es’hail2

A priori: Click on images to enlarge in new tabs.

Introduction.
Es’hailSat-2 launch is scheduled for 12 October this year. Look here.
It is planned to carry a first geostationary payload for amateur use in history!

However . . . the AMSAT contribution to this payload is (kept?) vague and
it’s a mystery to me what AMSAT hardware actually will be shot into space.

Es’hail2 is a commercial (broadcast) satellite. So . . . they have plenty of
Ku –> X band transponders and almost certainly also Ka –> K band transponders.

Uplinks for Ku –> X transponders are in the 12-14 GHz range, downlinks around 10 – 12 GHz.

So, it could be well that the AMSAT 3cm allocation (10.45 – 10.50 GHz) ‘bleeds through’
a lower transponder edge. This may suggest that the only ‘additional AMSAT payload’
may be a 2.4 GHz receiver with antenna. Baseband output (let’s say 10 MHz wide)  of the
2.4 GHz receiver may be upconverted to around 12 GHz, simulating a ‘normal’ earth uplink
which is linked down around 10.5 GHz. How to deal with AGC issues is not trivial.

The 10.5 GHz downlink is divided into two portions: 250 kHz narrow band and 8 MHz
‘wide’ band for DATV use or other amateur broadband application.

Anyway . . . the scheduled 250 kHz ‘narrow band’ transponder from grounds perspective is:

1. Uplink is centered around 2400.175 MHz. Polarisation RHCP and EIRP ca. 31 dBW.
2. Downlink is centered around 10489.675 MHz. Polarisation is LV and G/T ca. 13 dB/K

Thumbs up for the people who did the lobby, VERY fine job!

My quick & dirty idea for a 10.489 GHz downconverter involves a satellite PLL LNB.
These PLL LNB’s are cheap and own reasonable noise figures (NF) around 10.5 GHz,
which is somewhat below their target band around 10.7 – 11.7 GHz.

Although HDTV requires more stability, PLL’s inside these LNB’s are not stable enough
for narrow band (e.g. SSB) operation. However, there are solutions in order to significantly
improve LO-stability in these LNB’s to comply with narrow band usage.

One of these methods involves injection locking the PLL-reference signal into
the LNB. Most LNB’s use a 27 MHz (crystal) reference. So, the trick is to ‘overrule’
the built in oscillator with a more stable 27.000000 MHz signal.

Contrast to other AMSAT satellites, Es’hail2 will not contain doppler due to its
geostationary slot. Normally you would transmit a signal on the uplink frequency and
listen to the downlink with a separate receiver in order to determine doppler offsets
because the satellite is moving.

Now the satellite hangs at a given geostationary location (26° E) so it’s merely a
giant repeater with some delay and a gigantic repeater offset. ‘Shift’ is -8089.5 MHz (!)

This means when you’ve adequate frequency precision on earth, one transceiver suffices!

Because most uplink contraptions will be build with a block upconverter (aka BUC),
locking the upconverter LO to a frequency standard is pretty straight forward.

The downlink converter, i.e. the PLL LNB, has to be locked to the same frequency standard.

My idea is to build a ‘box’ with all necessary equipment inside, i.e.:

1. 432/144 MHz –> 2400 MHz upconverter (ca. 10 – 20W ‘at the feed’)
2. 10.489 GHz –> ? MHz downconverter
3. GPS disciplined oscillators (GPSDO) for 10 and 27 MHz.
4. Power supplies, switching stuff etc.

This box has to be placed near/under the (dual band, i.e. 2.4 GHz LHCP, 10.5 GHz VP) dish feed.
(Note: to achieve RHCP you need to build a LHCP dish feed!)

Approach.
10 MHz GPSDO’s have been extensively published (Google on it). The idea is to lock
the LNB 27 MHz reference oscillator to a 10 MHz GPSDO. Most LNB’s own LO’s at 9750 MHz.

This means that there is a 9750/27 = 361.1111 multiplication factor. Assuming the IF transceiver is
stable enough, to have e.g. 10 Hz frequency accuracy at 10.5 GHz, the LNB LO needs a precision
around 1E-9. This means the 27 MHz reference signal needs 1E-9 / 361 = ~ 1E-12 (!)

Whether I can reach 1E-12 precision with junkbox parts has to be found out.
The proof of the pudding is in the eating, so I scraped my junkbox and started building.

Below my initial circuit sketch is depicted.

The circuit above is a schoolbook PLL example. A GPS receiver provides a locked 10 kHz reference
to an XOR phase detector, followed by a loop filter controlling the 10 MHz TCXO phase/frequency.
TCXO output is levelled to TTL and divided by 100 (HC390) and 10 (4017) to deliver 10 kHz.

Of course two HC390′s could be used, but my junkbox revealed only one piece. The 4017 carry
output (pin12) is used to deliver a 50% duty cycle 10 kHz square wave.

Et voila . . . the loop is closed and (hopefully ; -) locked.

Next idea is to build a 27 MHz VCXO. Its output has to be levelled to TTL and subsequently
divided by 27 (= 16 + 8 + 2 + 1) to obtain 1 MHz. Division by 27 is done with a binary counter.
In the initial circuit sketch a 4024 was selected. However, I discovered my junkbox had more
4040′s than 4024′s. A 4020 or HC590 also works.

Although the resulting 1 MHz duty cycle will be (27-16)/27 = 40%, my initial approach
is to ‘stretch’ this to 50% by means of a D-flipflop which is clocked on the positive edge.

Side effect is that the 1 MHz signal is divided by 2 to deliver 500 kHz.
Therefore the 1 MHz signal point from the 10 MHz GPSDO passes through an identical D-flipflop.

Both 500 kHz signals are fed into an XOR phase detector, followed by a loop filter to provide
the necessary control voltage to discipline the 27 MHz crystal oscillator.

When 27 MHz VCXO lock yields nice results, discarding the two D-flipflops (74HC74)
adheres to a more minimalistic and junkbox approach ; -) Perhaps something to try later . . .

The PLL loop filter took quite some fiddling. The used TCXO can be slightly adjusted and its
VCO gain K (Hz/V) is relatively low. While experimenting, this meant l o o o ng locking times.
With patience I managed to get the loopfilter response and damping factor such that it seems ok.

I haven’t measured the 10 MHz signal on a spectrum analyser, so I haven’t the faintest clue on
phase noise and spurs. But.. what I certainly know, there is a spur around 10.140.000 Hz . . .
Very likely this spur is derived from the 10 kHz reference and super imposed on the 10 MHz signal.

From the left picture above it can be clearly seen that the 10 MHz TCXO is crying for discipline
after a cold start with no GPS fix. After the 10 kHz reference is locked to GPS the TCXO is disciplined.

The right picture above shows a cold start with a valid GPS almanac.

Next, build the 27 MHz oscillator and lock it to the GPSDO. More to come !

 

April 4th, 2016

Icom IC-402 repaired

A priori: Click on images in this post to enlarge in new tabs.

Introduction.
“Going back in time on the sound of the nation it’s a flash back back back … ”
Listen here to this popular jingle from the 70′s .

For whatever reason I’m in a repair mood lately. I got fed up looking at equipment
‘to be repaired’ but never did. Sometimes hobby is similar to work. You’ve to do
things which are dreadful. But these dreadful things have to be done by someone (me).

What is the issue. I own two IC-402′s of which one became defect around 7 years ago.

Say what? IC-402? Are you nuts? Well . . . yes and no. However, bear in mind these
old fashioned IC-402′s deliver our VHF/UHF/SHF contest team victories since >30 years.

First, let me post some pictures. Below left is a picture taken during the March 2008 contest.
IUD (Icom Under Discussion) is located on the right, at that time apparently working.
The left IC-402 is owned by my friend Ron PA3BPC / DL3BPC.

Below right is a picture I took around one hour ago, trying to win the RigPix contest ; -)


Left: 2x IC-402 active during a 70cm contest, right: IUD on my workbench.

Our contest team won innumerous 70cm contests with IC-402′s as driver and receivers.
This neat little rig sounds immaculate due to its remarkable low LO phase noise.
Yes, we tried other transceivers but everytime we went back to the good old IC-402′s.

And yes, these rigs are over 40 years of age. But . . . still outperform modern transceivers.
Our ‘last resort’ argument over the last 30 years towards sceptic persons is:

“If your transceiver is better than our IC-402, why don’t you win contests?”

100% of the time the sceptic remains silent (because he didn’t win nor does he own IC-402′s ; -).
Therefore, the proof of the pudding is in the eating. IC-402′s taste very well!

Oh yes, one secret is revealed now.
I modified the MF strip of my IC-402′s according to this document.

The issue.
I could hear noise, but that was all. Both RX and TX didn’t work.

Now . . . where to start. Look (and click!) on the images below.

Have you studied the enlarged pictures above? If yes . . . it’s a mess (which rhymes ; -)
But, this is ‘how it was done’ in the 70′s. At that time state of the art. In 2016 we frown our eyebrows.

Again, where to ‘start’? Well, first thing -after a ‘non power up failure’- is to
ascertain if all ‘frequencies’ inside a transceiver are present. After connecting a counter to CP2
it appeared the ‘band’ (crystal) LO worked fine (Google on the IC-402 manual to find out).

Next IUI (Item Under Investigation) was the VXO. My counter had some difficulties measuring the
VXO frequency. Tentative outcome was around 47 MHz, so it seemed to work fine.

Sometimes you need a little luck. My efforts to measure the VXO frequency caused suspicion.

Using an Ohm-meter the resistance across the VXO terminals appeared to be (close to) 0Ω.

Looking at the IC-402 circuit diagram this was weird. Could there be a shortcut? If yes, where?

Lets look at the relevant part of the circuit diagram below.

It appeared that the thin coax cable from the VXO to the 2nd mixer was shortcutted indeed.
However, in order to ascertain where, or to replace the cable would be a horrendous job!

With lots of fast movements I was able to dismount the thin coax from the mixer module.
Because the D1 and D2 cathodes connect AND face upwards it would be relatively simple to
connect a (new) VXO cable. Measuring these cathodes (D1, D2) revealed no shortcut towards GND.

I also disconnected the connector at the VXO side.
When I tried to pull out the small thin cable for inspection it was ‘firmly’ stuck.

Like I said, sometimes you need some luck when repairing equipment.
It seemed the VXO cable was squeezed between a nut washer and the chassis!
Now . . . what is this for a defect ? This fault must have been inside this IC-402 ever since !

Below a close up picture of the solved issue.

After releasing the cable from its nut washer I fiddled a little and the shortcut disappeared.
Inspecting the cable damage with a magnifying glass revealed the cable could be rescued.

Quickly the VXO cable was resoldered and a 432 MHz signal was applied to the antenna input. Presto!

I spent some time to trim the receiver. The result was a ‘non’ Minimum Discernible Signal (MDS).
In other words, I could easily hear the lowest signal generator level (-140 dBm) from the speaker
and didn’t bother to insert an attenuator in order to measure the full Monty.

Long story short, this IC-402 works flawlessly and is ready to be used in 70cm contests : -)

April 3rd, 2016

Yaesu FT-780R revitalisation

Work in progress… read on …

A priori: As always, click on images to enlarge in new tabs.

Introduction.
I worked at the Dutch Radio Communications Agency and periodically
administrative obsolete equipment was offered to staff members before it was destroyed.

Nowadays, due to governance issues this seems impossible … anyway …

The procedure was co-workers could subscribe to a list. After a while
you were informed whether you wanted to buy the item for a scrap price.

Around 1998 I got my Yaesu FT-780R for around 10 Hfl (ca. 5 Euro nowadays).

This FT-780R was used in our monitoring station (NERA) ‘to enforce amateur satellites’
(I was told). I was also told this FT-780R was ‘custom modified’ so that it was not able
to transmit, in order not to damage other sensitive monitoring/receiving equipment.

After all, NERA was a monitoring station, not a transmitter site ! ; -)

When I got this FT-780R the receiver worked okay, but when you pressed PTT
the processor crashed, resulting in ’8888888 88′ on the display.

For whatever reason I left this FT-780R in a box for more than 15 years . . . until recently.

Below the bottom cover of my FT-780R is depicted.
I think not many radio amateurs use equipment used and owned by their enforcement agencies ; -)


Rationale.

A few months ago some friends in my neighbourhood decided to build a linear transponder using
2320.7 MHz in and 432.7 MHz out, BW = 15 kHz. I own two IC-402′s but these lack crystals for 432.7 MHz.
Despite I have a FT-857 I thought it would be a nice idea to have a dedicated rig for this transponder
in conjunction with a 2320 <–> 432 MHz transverter.

So… I fetched the old FT-780R from my storage box and decided to ‘remodify’ it.

First I looked up the circuit diagrams on the internet in order to investigate these ‘secret
non transmit’ modifications. I found a ‘user manual’ PDF, but circuit diagrams were split.

I ‘reassembled’ the circuit diagrams into one piece with a pair of scissors and took
pictures of the results as depicted below.


‘Remodification = repairment’

My assumption this FT-780R was modified in order NOT to transmit seemed valid at first glance.
My connotation of the word ‘modification’ involves (some degree of) reversibility.

Along the PTT line an ‘extra wire’ was hooked up to the processor board.
Removing this wire eliminated crashing after PTT.

RF output was absent but . . .  I could hear myself on a nearby receiver.  Promising!

Optimistically I started to inspect the final stage, consisting of a Mitsubishi M57716 module.
The relevant part of the circuit diagram is depicted below.

In/nearby the final amplifier stage I noticed three ‘issues’ (refer to right picture above):

1. The antenna relay did not switch per PTT because the ‘RL’ wire was dismantled.
2. Power supply leads of the last amplifier stage were removed both inside and outside.
3. Q2 (2SD235Y) was missing (??) and bridged so PO CONT is forced to 13.8V (??)

Issues #1 and #2 were solved as depicted below.


Issues #1 (left) and                                  #2 (right) solved.

Issue #3 is not a real issue concerning output. It  eliminates the function of the LO/HI power
button on the front. Of course I hadn’t a 2SD325Y but inserted a BD139 in the small pertinax board
next to the 7808 (Q1).

Anyway,  issue #3 is a very queer ‘modification’ in order NOT to transmit . . . (?)

After solving these ‘issues’ I pressed PTT in FM mode . . . .  NO output.
Perhaps the RF module was damaged or received no drive?

Indeed, I measured no drive, so . . .  further investigation was necessary.

The driver for the M57716 resides inside the PLL unit.
Relevant part of the circuit diagram is depicted belowt.


.                                           FT-780R M57716 driver stage.

First inspection of the driver stage didn’t reveal something strange.
Relevant power supply voltages (13.8V and TX 8 Volt) were there, but no drive output.

After careful inspection I couldn’t believe my eyes . . . .  Q05 (2SC2026) was ‘missing’ !! ?

Remember my connotation for the word ‘modification’ ?
For me a ‘modification’ owns a certain degree of reversibility.
In my perspective one of my ex colleagues from the technical department stripped
Q05 from the PCB, and very likely landed in the waste bin !!

I know lots of ex colleagues read and enjoy this blog.
So. . . when you read this and it was you, or you know who it was, contact me?

Being ‘in full swing’ I dismounted the PLL unit for inspection and insert a new Q05.

Below pictures of the ‘missing’ Q05 and dismantling of the PLL unit are presented.

I could have soldered a new Q05 on the top side of the PCB but I wanted a ‘clean repair’.
Of course I hadn’t a 2SC2026 so I chose a good old BFR90 instead. And old it is, 41 years !
Below pictures of the bottom side of the PLL unit are presented. I reckon very few people have seen this side ; -)

After reassembling the PLL unit I measured 5.5Vpp RF @432 MHz over 46.4Ω with a decoupled OA91 germanium diode.
This means corrected around 5.8Vpp, resulting in (5.8/√2)² / 46.4 = 362 mW drive (which is too much btw).

I reconnected the drive cable to the M57716 unit and gave PTT in FM mode . . . . NO output : -(

Thus, very likely also the M57716 module is defect! See pictures below.

At this moment a decision had to be made to replace the M57716 module. In conjunction with
a 2320 MHz transverter replacement is not really necessary. I can route the drive signal from the
input (pin1, right) to the output (pin5, left) of the module with a small coax.

On the other hand, it is elegant to restore the FT-780R for standalone work. So, I ordered a M57716 from Ebay.

Awaiting its delivery . . . more to come, stay tuned!

 

 

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