[R-390] Detector

[Bob Camp]

Hi

What ever you are using to measure the output of the filter is a detector.

A fairly good approach to filter testing:

Input side (option 1):

1) Coax (any length) from the signal generator over to the test setup. 
2) Mini Circuits AD4-6T transformer on the end of the coax
3) Terminate the transformer in 200 ohms
4) 2 x 50 K (one from each side of the transformer) to filter input
5) ~120 pf between the ends of the 50K's
6) filter input to the same junction 

Input side (option 2):

1) Coax (any length) from generator to the test setup.
2) Terminate coax in 50 ohms
3) 100K to from coax to filter input
4) ~ 120 pf cap to ground on filter input

Output side (option 1):

1) 100K across the filter output
2) ~ 100 pf across the filter output
3) High impedance scope probe across output
4) Scope probe to scope input. A TEK 2465B would do nicely. 
 
Output side (option 2):

1) coax (any length) to 50 ohm scope input
2) AD4-6T 
3) terminate transformer in 200 ohms
4) 2x50K (one from each side of transformer) to filter output
5) ~ 120 pf between ends of the 50K's 
6) Filter output to same junction

Output side (option 3):

1) Boonton 92 RF Milivoltmeter, probe to filter ouput
2) Cap ~ 120 pf across filter output
3) Run on lowest two scales on meter, adjust generator to stay in range

Output side (option 4):

1) BC-453 on filter output
2) Cap ~ 100 pf across filter output
3) AM modulate the signal generator, measure audio out of the radio

In all cases the approach is the same:

1) Remove filter and caps
2) Replace filter with shorts
3) Measure level, that's your zero db point
4) Put the filter and caps back in
5) peak the caps
6) Measure the level


Any input option can be used with any output option. 

The key point is to keep any high impedance leads very short. Stray capacitance is a problem. 1/8W carbon comp resistors are the best thing to use for the resistors. Ceramic or silver mica caps are about the only way to go for the caps. You want a solid ground plans under the filter. PCB material is the normal way to take care of that. Short leads on all the caps and resistors are a really good idea. 

Bob
 
On Feb 23, 2013, at 10:20 PM, quartz55 <quartz55 at hughes.net> wrote:

> Bob,
> What do you mean 'detector'?  I've worked on filters/isolation switches before and granted I'm not using good 'bypass' circuitry, but I guess we'll see when I get the thing back together.  I'm no expert here.  The thing did exibit very high loss in the R390A RX.
> 
> Could you give me a good example of what the test circuit should look like, in 'real components' with 'real test equipment'?  Not j factors.
> 
> Dave
> N3DT
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