Marconi 2383 YIG output

[Andrew Holme]

Front end gain of a Marconi 2383 spectrum analyser I bought on eBay is about
6 dB less than it should be and the noise floor is about 6 dB higher as you
might expect. I suspect a lack of LO drive. The 1st mixer drive level is
specified as +17 dBm nominal. Between this and the YIG there is only an
amplifier and a power splitter. Annoyingly, the service manual does not
give signal levels for all points in the drive path and is
self-contradictory on stage gains; however, I think the amplifier is 6 or 7
dB and the splitter is -3 dB so the YIG output needs to be +13 dBm. I
measured it at +10 dBm. According to the article below, YIG outputs vary
between +10 and +15 dBm so mine is at the bottom end of this range. Do YIG
outputs decrease with age? Mine is 20 years old. Any advice appreciated.

http://www.vhfcomm.co.uk/pdf/A Simple Approach tyo YIG Oscil.pdf

 

[Alan Peake]

Front end gain of a Marconi 2383 spectrum analyser I bought on eBay is about
6 dB less than it should be and the noise floor is about 6 dB higher as you
might expect. I suspect a lack of LO drive.

Doesn't sound like the LO drive. If the drive level is 3-4 dB below
the nominal, conversion loss will only increase by less than a dB for
most mixers. Sounds more like a pre-amp failure if it has one - or some
sort of unwanted attenuation in line with the input.

Alan

 

[Peter Baltus]

Doesn't sound like the LO drive. If the drive level is 3-4 dB below the
nominal, conversion loss will only increase by less than a dB for most
mixers. Sounds more like a pre-amp failure if it has one - or some sort of
unwanted attenuation in line with the input.

Alan

One maybe less obvious source for such attenuation might be the connector.
I've seen several dB of attenuation at several GHz in the past when people
mixed up 75 Ohm and 50 Ohm N-connectors. You can damage the connector that
way, because the center pin is thicker but will fit if you try hard enough
;-) After that, there might be a small airgap if you use the proper
connector. This airgap (or reduced spring strength of the connector) can
cause several dB of attenuation.

Peter