Protecting the 1st Mixer in a Transceiver application on transmit

[Xnke]

So I am working through bits of the radio I've been puttering with for a few years now, and while the low-power part of things is done and works quite well, it's time to add the final amplifier, the low-pass filters, and the antenna transmit-receive changeover relay.

The relay isn't a big deal, I thought it was but turns out it's not too terrible. The final output should be about 50W of transmit power, and I need to make sure that on transmit, there is enough isolation through the disconnected relay contact that the input of the mixer doesn't get damaged. It *shouldn't* be a problem, because the first mixer is actually going to be driven as the balanced modulator during that time and should have an audio frequency signal to work with.

But, testing the TRX relay (which is the same Axicom D2-sized signal relay as the rest of the receiver), the isolation between the selected and deselected contacts is about 43dB. 50W is 47ish-dBm. That comes out to about 2.5mW of power leaking into the mixer input. There's another 1-2dB of loss through the preselect filter, so it's not a LOT of power that's going to get through.

I could use a second relay on the mixer's input to add another 43db of attenuation, but since it's a Mini-circuits ADE-1ASK (Or I could use Mini-Circuits RMS-2) is it even going to be a problem? The rated RF power for the ADE-1ASK is 50mW, and the RMS-2 is 200mW.

It doesn't *seem* like this would be an issue power-wise, but signal-wise, would it be enough worry about it causing issues?

Essentially, the TRX relay is switching the antenna port between the transmit LPF board, and the receive BPF preselect board, and the receive BPF's are feeding straight into the mixer's IF port.

[radioactive]

You might consider adding a pin diode shunt to the rx path during transmit if you are concerned that it could be an issue.

[German_EE]

A number of things may help here:

1. If your receiver has a switchable attenuator then enable this during transmit.

2. Have a large relay to switch your antenna from transmit to receive, this can be as slow as you want so long as you take account of the delay in your QSK switching.

3. Have a small reed relay directly across the receiver front end that activates as quickly as possible to protect the front end.

[KJDS]

I'd use a few PIN diodes instead of the relay. The only real issue is that you'd need about 70V to ensure that the RF signal didn't turn them on when you want them off.