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Old 8th Nov 2013, 12:02 am   #17
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Marconi 2022C - Stopped working below 62.5 MHz

The 100nF caps aren't needed in the sig gen because there are already some blocking caps in the circuit.

In the case of IC2 I'm not sure it matters about the 75R vs 50R. It looks like it is fed into the prescaler in parallel with a 51R resistor. I think the input impedance of the prescaler is quite high so I optimised the buffer circuit for a 50R load.

In the Mi2022C you can see IC2 has a 100R resistor in series at the input and I suspect that this is a crude way to prevent IC2 from being overdriven and the series 100R at the input may also provide extra reverse isolation and it also kind of prevents IC1 from being double terminated.

The circuit I gave above really is just for IC2. It won't work very well at higher frequencies because I'm sure the performance will degrade due to the PCB layout.

When I looked at this last time I had to add inductance in a couple of places to get good performance at 500-1000MHz and I did wonder if the OM345 has some little inductors inside somewhere to help perk up the performance at the top end.

So the simple circuit I gave really is only suitable for IC2 down at 160MHz

I'll see if I can find my original simulations because I also included the three tone tests for the DIN IMD spec in the datasheet. I couldn't quite meet this spec the last time I tried but it probably doesn't matter for most of the locations in the Mi2022 because all but one appear to only have a cw signal. One of them is inside the ALC loop on the high range and this one will see AM modulation.
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 8th Nov 2013 at 12:17 am.
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