Testing a transceiver without blowing out my Spec-an or sig-gen - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: September 07, 2014, 12:05:54 pm
content: Hi there,One for the RF experts! (Or at least those who know more than little old me, which is... probably just about anyone)I've got my hands on an old Tait T454TR MPT1327 trunked transceiver for a long-since-decommissioned service. My plan is to modify the firmware to allow it to work as a data modem (I have another project where FFSK data would work really nicely) and voice transceiver, and retune the VCO to handle the 136-174MHz amateur band. But before I do that, I'd like to see it working "as is", just for kicks (and because I'd rather like to see an MPT trunked radio system in action).I've found an open-source TSC controller (SoftTSC,https://paulbanks.org/projects/softtsc/) which could handle generating the control channel. Frequency-wise, it's a bit like a repeater - an "uplink" channel which the TSC listens on (and which transceivers may talk on when the TSC grants them a timeslot) and a "downlink" which the TSC transmits on. This conveniently means I don't need to turn the RF out from the signal generator on and off (key-up) to allow the transceiver to speak. In theory, anywayMy plan is to set SoftTSC up on a PC, wire the audio-out to a Marconi 2022E signal generator (currently in transit) and use my Advantest spectrum analyser's FM demodulator mode to receive the signal, as I don't have a receiver which covers the T545TR's frequency band.That leaves RF connection - the biggest problem being the disparity between transmit and receive power. I only need a tiny output from the SG for the T545TR to detect it (something like -114dBm for 20dBm SINAD), but said radio spits out a whopping 25 Watts (whether it can do this into 50 Ohms is debatable; at 13.8V in, the most you could put over 50 Ohms is 3W per Ohm's Law... so how they can do 25 is a mystery to me. It could in theory dump that into a 7.6 Ohm load, though). I can tune this down if need be, but the radio service manual has a big warning about taking it below 5W (I guess the PA goes non-linear or becomes unstable?).I'm absolutely certain that shoving 25 watts up the front end of a spectrum analyser isn't going to be the least bit good for it, and I doubt the signal generator would appreciate it much either. So how would I go about connecting the radio up without risking my test kit?Initially I started thinking about using power splitter-combiners, but I couldn't think of a safe way to do it. As soon as the radio transmits, the combiner becomes a splitter, and both the SG and SA get about 12.5W each. Still not good.My initial research led me to duplexers -- which would seem to fit the bill. Two notch filters prevent the RX from seeing the TX, and vice versa. The problem being, they're frequency dependent (not to mention expensive) and I can't find one in this frequency range. So it'd have to be a custom job, and the cost of the copper pipe alone was a rather sobering thought.I've seen radio test sets which can do this -- the Marconi 2955 series, a few equivalent HPs... But that's a piece of test gear designed for that sort of use. Is there anything equivalent that I can do with the kit I have?Thanks,Phil.

author: Yago
author_rank: Frequent Contributor
timestamp: September 07, 2014, 12:23:16 pm
content: Could you use a dummy load on the output of the transmitter, and then just loop a probe in proximity to the dummy load.Enough RF should be around there to be picked up clearly.

author: philpem
author_rank: Frequent Contributor
timestamp: September 07, 2014, 12:37:15 pm
content: Quote from: Yago on September 07, 2014, 12:23:16 pmI could, but if I understand correctly, that only solves the TX out problem. It leaves me with the problem of somehow coupling the "control channel downlink" (the output of the SG) into the receiver.There's a handshaking procedure the radio does with the TSC - both channels (up and downlink) need to be active for that to work.Essentially, it's:TSC says "I've got a spare timeslot, would anyone like it?"Mobile switches to the uplink and keys up, then replies "Hello, I'm unit 123456, can I register with you?"Mobile switches back to the downlink and starts receiving.TSC replies "Hello 123456, I know you, and it seems your control bits are correct! You're now registered on the network."

author: Yago
author_rank: Frequent Contributor
timestamp: September 07, 2014, 01:06:22 pm
content: Ah right, same as you I have seen the radio engineers use test sets for this purpose.Maybe make a dipole for the signal gen.

author: German_EE
author_rank: Super Contributor
timestamp: September 07, 2014, 02:05:14 pm
content: What you need is something called a Power Tap between your transceiver and your dummy load, please see the following link and then scroll down about 75% of the page:https://sites.google.com/site/lofturj/ad8307-power-meter25 Watts is about 44 dBm which should NOT be fed into a Spectrum Analyzer but the output from the power tap will be about 4 dBm or 2,5mW which is quite safe.Coupling the signal generator into the receiver is more complicated and you have two choices:1) Fit the test output of your power tap with a 50-ohm splitter, one port goes to the spectrum analyzer and the other port goes to the signal generator.Please make sure that feeding 4dBm into your signal generator will not be a problem before doing this!The signal from your generator will be -40 dB down but this will not be a problem for the receiver.2) Couple the signal generator to the receiver input after the transmit/receive switch.

author: SeanB
author_rank: Super Contributor
timestamp: September 07, 2014, 02:40:57 pm
content: You need a directional coupler. so that you get the power fed from the transmitter to go to a dummy load, and the 2 ports on the coupler give you a sample of forward and reverse power. Bird meters are cheap enough, and some have 2 outputs to give a forward and a reverse sample. The forward one goes via a few attenuators to the SA and the reverse is used to couple the output of the signal generator to the unit.The way these transmitters get 25W out of a 12V supply is simple, as they use a transistor PA, which typically has a need for a 2R output load or so, and use an autotransformer ( inside it will be a few turns of thick wire in a coil on a ferrite core with around 3 times as many thinner windings wound around it) to transform the impedance to match 50R transmission lines.

author: philpem
author_rank: Frequent Contributor
timestamp: September 07, 2014, 08:20:51 pm
content: Quote from: German_EE on September 07, 2014, 02:05:14 pmVery nice -- I can probably build that and the wattmeter from parts I have on handQuote from: German_EE on September 07, 2014, 02:05:14 pmGoing by the front panel on the 2022E, it can handle 25W reverse power. According to the manual, it can protect itself from reverse power of up to 50W by switching the output with a relay. Resetting said relay involves a GPIB command or a magic sequence of keypresses... I don't intend to try that protection circuit, but I don't expect 4dBm to do much to it.Quote from: German_EE on September 07, 2014, 02:05:14 pmI'll keep that as a backup option - it looks like there'll be a bit of a wait to get the power splitter/combiner. I really need to find a UK Mini-Circuits distributor...Quote from: SeanB on September 07, 2014, 02:40:57 pmI've got a Minicircuits directional coupler on the way, but sadly it's only the 3-port version, and only good to a few watts. Four-port couplers seem to be quite rare, especially when you want to put a fair bit of power through them.Unless I could build one, but that's a whole other line of inquiry.Quote from: SeanB on September 07, 2014, 02:40:57 pmThat... makes so much sense. That'd be an impedance matching transformer then? Neat. And makes total sense - you can't get 25W over 50R from 13.8V, so step up the voltage a bit?Thanks for the explanation!Thanks,Phil.

author: Stonent
author_rank: Super Contributor
timestamp: September 07, 2014, 10:36:30 pm
content: This may help with the sampling part:

author: KJDS
author_rank: Super Contributor
timestamp: September 08, 2014, 07:40:36 am
content: I'd use a 30dB attenuator and a directional coupler. Don't rely on the sig-gen self protection. It should work, but so should the airbags on your car.You can make a poor performance directional coupler by stripping a couple of inches of the outer conductor of some thin coax and pushing this inside the outer conductor of some thick stuff. Solder the two outers where they meet.

author: philpem
author_rank: Frequent Contributor
timestamp: September 08, 2014, 09:19:54 am
content: Quote from: KJDS on September 08, 2014, 07:40:36 amSo - correct me if I'm wrong - basically shove the dielectric and shield of a piece of RG174 inside a piece of RG58? Something like this?############################### RG58 shieldxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx RG174............................... RG58 dielectric----------------------------------- RG58 core............................... RG58 dielectric############################### RG58 shieldSeems a bit how-you-doin'I'm not above etching PCBs or winding small RF transformers to do a proper job of it. Probably got most of the stuff on-hand, actually...Also I'd need to get my mitts on a 30dB attenuator... I've got a dummy-load on order (good to 60W I think, complete with free BeO core and "thou shalt not disassemble" warning stickers) but an attenuator might take a few days. I'm off to the National Hamfest though so who knows what I'll find there?

author: KJDS
author_rank: Super Contributor
timestamp: September 08, 2014, 09:40:07 am
content: Quote from: philpem on September 08, 2014, 09:19:54 amYou need to strip the outer conductor from the RG174. It won't be brilliant, but it will work as well as all but a very carefully designed and constructed coupler of other forms. If you stide two in, at different points of the main line and pointing in different directions then you could connect one to the sig-gen, the other to the spec ana and terminate the main line with the dummy load. Just check the coupling factors before you transmit through it.