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