Comment by mikewarot
6 hours ago
Having done repairs on a ton of older service monitors, and having my hand in SDR, this is amazing! There's a huge amount of machining and hardware to make a service monitor, but with desktop CNC, cheap PCBS, and jelly bean components that go into the 10s of GHz, the sky's the limit.
I'd add a heat sunk 100 watt DC-5Ghz 20dB fixed attenuator, 0-110 dB switched attenuator, 5 ghz low pass filter, 7-12 Ghz local oscillator/mixer up/down converter, and it would make a small, very high performance service monitor good to 5 Ghz. 8)
Service monitor?
A Communications service monitor is a tool for servicing radios. Until recently, most large organizations used VHF/UHF FM radio for communications on dedicated channels. A service monitor is a tool that can usually absorb the full output power of these radios while measuring power, frequency, distortion, etc. They can also generate a variety of signals for aligning receivers.
Most of the older, affordable models go from almost DC to 1 Ghz. I've got an IFR 2947a, which goes up to 1.05 Ghz.
Here's an example from ebay.
https://www.ebay.com/itm/168770412399
Here's a specification sheet for the one I have, a pdf
https://www.testmart.com/webdata/mfr_pdfs/aerofl/2947a.pdf
Why 4ghz?
I've edited it, but generally you don't want to have problems with mirror images getting through and creating spurious responses. Real world RF filters don't have infinitely steep edges, so initially I figured a 4 Ghz low pass filter would have enough loss by 5 Ghz to get the job done. I revised to to a 5 Ghz low filter, and a 7-12 Ghz Local Oscillator to give the filter an easier job. Also, the lower the swing in percentage terms for a VCO, the easier it is to work with.
So the signal path in receive mode would be:
N connector / 20 dB 100 watt pad / Switched attenuator / Low pass filter / Front End diodes / Mixer / Router board for DSP
Alternatively, there's usually a second path that skips the attenuator, with a lower maximum power level.
The reverse path would work for generating signals from DC (almost) to 5 Ghz.