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Author Topic: Transceiver Design: switching between Tx and Rx  (Read 368 times)

MACLEINN

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Transceiver Design: switching between Tx and Rx
« on: January 18, 2022, 02:59:21 AM »

Hi,
I'm looking for feedback from you seasoned home builders on the subject of switching between receive and transmit in a transceiver.  Some background: I'm building a mono-band SSB (20m) transceiver, it's a pretty standard single-conversion superhet design.  I've decided I'd like to share the IF strip (IF amp + crystal filter + IF Amp) between transmit and receive, which complicates the switching slightly.

The switching arrangement simply gates the signal around the two mixers (VFO/BFO) and IF strip using RF dpdt relays.  I have 3 small signal relays and one larger antenna relay. Here's a picture of the setup:

https://imgur.com/F3PzUtM

Notice that when the transceiver is switched to RX, the input to the transmit amplifier chain is O.C.  This seems to be a flaw in my design, the input to my high-gain output amplifier stages is effectively floating.  It then picks up noise; small at the beginning, but by the time it goes through all the gain stages and the P.A. sections it's packing quite a punch.  Since the antenna relay has disconnected the load from the P.A. in receive, this energy is going nowhere and I'm guessing that's not a good situation for my MOSFET finals!

Looking around, I find another author with a similar approach.  This one by JF1OZL, a builder whose work I appreciate and I hope he doesn't mind that I highlight his work to make a point.

http://www.next.gr/uploads/685-54d685.gif

Notice in this design the output stage is wired to the RF amp output on RX.  To avoid the P.A. pumping out a signal during RX, he grounds out the P.A.  I'm not so sure about this as an approach, but I'd like to hear your opinion?

So, anyway after all that I have a few options.  My favorite is to include an additional relay that simply kills the TX power rail on RX.  I'll probably need a switch like that anyway for some SWR safety circuitry I intend to include later, so it's not a big deal.  The only disadvantage I can see is that it takes a finite time for the P.A. sections to come up to temp and I'm not sure what effect this will have on the output.  Another option is to ground the input of the P.A. chain, which would obviously eliminate the floating noise.

I'd appreciate your feedback and stories of woe if you have any.  Kind regards. D.
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K1KP

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Re: Transceiver Design: switching between Tx and Rx
« Reply #1 on: January 18, 2022, 04:45:42 AM »

The noise from the PA will also probably get coupled into the receiver through the output TR relay poles, degrading your noise figure.

Sounds like you're on the right track with any of these approaches. Here is two cents worth of ideas:

1. You could use an additional pole on an existing relay (DPDT instead of SPDT) to do the switching. Saves space/power.
2. You would prefer to switch a low-level signal to control the PA instead of switching the DC, because the DC has to be higher current and the other reason you noted about PA coming up to temp. Could also just disable the PA by removing the bias voltage.
3. Might consider a diode switch to ground the input to the PA. Forward bias to connect for TX, reverse bias (series element) to disconnect. Add a shunt element for zero signal input to the PA. Can be controlled with a logic signal, takes up less space than an additional relay.


-Tony, K1KP
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G4AON

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Re: Transceiver Design: switching between Tx and Rx
« Reply #2 on: January 18, 2022, 05:47:28 AM »

You can copy the timing circuit from my SSB transmitter. It is based on the MK 2 Universal QRP TX from QST, with improved timing.

I haven’t gone down the route of a transceiver, but the circuit might still prove useful.

https://www.qsl.net/g4aon/g4aon_ssb_tx/

73 Dave
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MACLEINN

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Re: Transceiver Design: switching between Tx and Rx
« Reply #3 on: January 18, 2022, 06:24:00 AM »

Thanks for the feedback guys.
@K1KP Great points Tony.  Based on your comments, I'm starting to favour the idea of controlling the input to the PA to solve this particular issue (although I would like to have a switch that kills power to the tx section to save on battery during mobile use).  I particularly liked the diode switch at the input.  In fact, I wonder if a simple high ohm resistor to ground might be useful, accepting that it will reduce the signal somewhat and increase the noise?  What "shunt element" were you thinking of exactly Tony or was that a diode also?

@G4AON Wow! Nice work. I have started to replace my manahattan style layout with pcb's now that I've proven some aspects of the design, but your boards look great, well done.  I'll need some time to digest your architecture in detail, but looking at the block diagram am I right in thinking you have a separate Rx and Tx antenna ports?  Also, are you controlling the PA bias from within the Tx control board?  Is the bias off during Rx?

Regards. D.
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G4AON

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Re: Transceiver Design: switching between Tx and Rx
« Reply #4 on: January 18, 2022, 07:13:26 AM »

I use a single antenna port, the TX routes the antenna to the RX, which does allow for a separate RX antenna if needed. I’ve used an external change over toggle switch to swap between the TX antenna and an active antenna.

The PA bias is switched off on RX. See Tr6 on the control board.

73 Dave

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WB6BYU

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Re: Transceiver Design: switching between Tx and Rx
« Reply #5 on: January 18, 2022, 12:55:49 PM »

I would start with a pair of switched lines, one with
+12V (or whatever) on Receive and 0V on Transmit,
and the other reversed.  Then you can use those to
enable / disable stages as needed.  For example,
you can shut off the bias to the amplifier stages in the
TX chain on receive.   These lines are easily generated
in your T/R circuitry, although in some cases you need
to watch the current capability, especially if generated
by logic gates.

There are several different ways to share circuitry.
Common ones include using the balanced mixer ports
on a SA612 or similar chip as two separate ports for
RX and TX, bilateral amplifiers (that operate in either
direction, depending on the polarity of the bias voltage),
and using CMOS switches like the 74HC4053 to switch
low-level RF in place of relays.

Shorting out the input to the TX chain is a good start,
but removing the bias from the stages reduces current
draw on receive and avoids some other potential
problems.

MACLEINN

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Re: Transceiver Design: switching between Tx and Rx
« Reply #6 on: January 19, 2022, 01:28:54 AM »

Thankyou for all the contributions so far, really appreciate your help.  I found some time to experiment yesterday and my attempts to ground out the tx chain had mixed results.  It seems if I use a short short (ha!), i.e., the tip of a flat-head screw driver, the noise signal (which is in the 14MHz range and about 1/5 of a watt) from the pre-amp goes away.  However, when I used an alligator clip cable (50cm in length) to do the same, it didn't seem to work.  I'm guessing the alligator lead is still acting like an antenna, even though it's grounded on one end.  I haven't completely ruled out oscillation in the 40dB pre-amp driver stage, but I've placed a hard-soldered ground at the start of the chain (just before the -6dB pad) as a test.  If that kills the spurious, then it's likely to be noise pickup.

On a different, but related topic, what would you expect to see at the output from the PA in tx mode with no mic input?  I'm getting a pretty strong carrier and as a novice to transceiver builds, I'm not sure what to expect?

Onwards and upwards! Regards. D.
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VK6HP

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Re: Transceiver Design: switching between Tx and Rx
« Reply #7 on: January 19, 2022, 02:41:58 AM »

There are a few hints that your transmit RF chain may be taking off on its own, or at least that it's on the verge of doing so.  I would go ahead and try grounding the input, and try it before and after your pad.  Normally, the pad itself would provide a decent enough termination, especially since your gain distribution looks reasonable.  It'd be worth having a close look at your layout and shielding, and at the de-coupling.  You've had a few good suggestions for the tx/rx switching but my own preference - assuming you're not counting every watt-hr - would be to leave the transmit chain energized for long-term gain/level stability, but obviously attack the RF pickup or instability issue.

You might get some more useful hints if you mention your construction style, or better yet, post a picture of the transceiver.  For example, I often use double-sided PCB construction with "islands" cut using a special tool; the other side of the board is ground (of course) and there are many through linkages spaced across the board.

I may have missed the RF output spec on your design but I'm guessing it may be 10 W or so (+40 dBm).  With a typical balanced modulator and SSB filter system, correctly adjusted, you'd get 40 dB or so carrier suppression, corresponding to about 0 dbm (1 mW) carrier output.  This could sound pretty loud on e.g. a receiver used for monitoring but if you're seeing appreciable levels on your RF power meter or scope it'd be another hint that your transmit chain (including possibly the early stages) is not stable.  However, I'd first have a go at carefully adjusting the balanced modulator and see where that takes you.

73, Peter.
« Last Edit: January 19, 2022, 02:51:50 AM by VK6HP »
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LA9XNA

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Re: Transceiver Design: switching between Tx and Rx
« Reply #8 on: January 19, 2022, 09:54:37 AM »

Have a look at how its dom on the BITX20 tranciever.
The design is pretty simple regarding TX/RX switching.
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WB6BYU

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Re: Transceiver Design: switching between Tx and Rx
« Reply #9 on: January 20, 2022, 04:57:45 PM »

Quote from: MACLEINN

... what would you expect to see at the output from the PA in tx mode with no mic input?...




The carrier is reduced by two steps. 

First, balancing the modulator.  Some designs have adjustments
for this, others don't.  If the carrier is too strong then you may
need to modify the circuit to adjust careful nulling out of the
carrier.  Other factors, such as construction methods and stray
capacitance, may also be a factor.

Second, the crystal filter.  The carrier should be a little ways down
the slope of the crystal filter, so it provides perhaps 6 - 10 dB of
additional attenuation (depending on how steep the skirts are).
If your filter is too wide, or the carrier is not properly adjusted
with respect to the filter bandwidth, then the carrier may be
stronger.  But first balance the modulator.


This all assumes that the driver circuits aren't oscillating all
on their own, of course.

MACLEINN

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Re: Transceiver Design: switching between Tx and Rx
« Reply #10 on: January 21, 2022, 12:54:40 AM »

Thankyou all! I did a little testing earlier in the week and it seems the transmit stage is oscillating under certain conditions.  When in receive, with the transmit stage grounded before the -6dB attenuation stage, and the PA unloaded the pre-amp driver and pre-amp stages take off on their own.  If I re-attach the 50 ohm load to the PA, the oscillation stops (there's still some noise, but nothing major).  I can't remember whether the same condition occurs at the pre-amp (with final PA disconnected, but with pre-amp loaded/unloaded into 50 ohms), but my vague recollection is that it doesn't.  I'll need to double check that.

Anyway, that's an issue to start with.  As for layout, well I'm embarrassed to post any images because I'm very aware that the layout is sub-standard.  I'm realizing that one of the disadvantages of modular design is that it becomes trickier to avoid leads flying all over the place (even if they are rg174 coax) during the integration stage, as has happened to me.  My plan now is this:
1) focus on the transmit stage; wire-up for transmit only and test/tune for reduced carrier without audio and no oscillations

2) consolidate some of the modular boards onto larger boards to benefit from a larger ground plane (and fewer flying leads) with some "glue" modules integrated into the larger board.  I reckon I can come up with some kind of daughter board strategy to avoid re-designing boards I've already printed as PCBs.

3) Double check my coupling and DC handling

So, plenty of work to do there.  On a positive note, the receiver is excellent with pretty astonishing sensitivity.  I think with a little bit of effort I can get the transmitter up to snuff and I'll be on-the-air!!

I'll keep you posted as I make some progress. Regards. D.
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VK6YR

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Re: Transceiver Design: switching between Tx and Rx
« Reply #11 on: January 22, 2022, 02:20:11 PM »

Hi D, In regard to carrier balance, if by chance you are using a SA602 or similar in this area, some time ago Allison KB1GMX suggested using a 500K trim pot on the input pins 1 and 2 with the wiper to ground. I have found this advice to be excellent and it has been possible to improve the balance considerably on my homebrew transceivers. Congratulations on your efforts.
Geoff VK6YR.
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G4AON

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Re: Transceiver Design: switching between Tx and Rx
« Reply #12 on: January 23, 2022, 01:49:25 AM »

Hello again D.
You mentioned the PA and driver taking off, needing a 50 Ohm load on the PA to stop it. The solution is to remove the bias from the higher power stage(s) and the supply from lower power ones. It might also help stability to use a small amount of attenuation between a driver and PA, the driver output and PA input are unlikely to be good matches for each other across a wide frequency range and a little attenuation helps.

The supply and bias need dropping shortly before the antenna changes over from TX to RX.

73 Dave
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