EMI test receiver HP 85422E sweeps wrong frequency (100MHz = 73MHz)

Hi

I recently bought a  EMI test receiver HP 85422E at an auction.

I have tested it, directing a single frequency into the RF input.

The gain is spot on, at most 0.5dB off. But the frequency is wrong.
If I enter 50MHz, I get 20MHz peak. If I then add 100MHz, it only increased the peak by 73MHz. It's consistent, every time I increase by 100MHz, only 73MHz is seen (generator has been checked on another spectrum analyzer)

I guess it works by sweeping a local oscillator, and generating an IF frequency.

What could be wrong?

Regards

Klaus

On 2023-09-01 15:43, Klaus Kragelund wrote:
> Hi
> 
> I recently bought a  EMI test receiver HP 85422E at an auction.
> 
> I have tested it, directing a single frequency into the RF input.
> 
> The gain is spot on, at most 0.5dB off. But the frequency is wrong. 
> If I enter 50MHz, I get 20MHz peak. If I then add 100MHz, it only
> increased the peak by 73MHz. It's consistent, every time I increase
> by 100MHz, only 73MHz is seen (generator has been checked on another
> spectrum analyzer)
> 
> I guess it works by sweeping a local oscillator, and generating an IF
> frequency.
> 
> What could be wrong?

Maybe a DAC reference voltage is off.  (It would need a nice linearized 
VCO for that to cause such a consistent error, but this is HP we're 
talking about.)

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com