[linrad] RE: Calibrating analog hardware

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hi Joe and all,

> What have people found effective as a signal source when doing 
> Linrad's procedure for calibration of analog hardware?  In 
> particular, has a tuned amplifier for the RF (or IF) frequency in 
> question been found necessary to get enough S/N?
The signal source needed depends on the dynamic range of your
receiver.

With a VERY good dynamic range it is possible to use a short
rectangular pulse on all bands up to 144 MHz
http://www.antennspecialisten.com/~sm5bsz/linuxdsp/pulgen/pulgen.htm 

This pulser gives 5V peak amplitude across 50 ohms so the peak
power is +27 dBm. Most RF amplifiers provide less output than this
so they provide attenuation while adding a lot of noise.

The pulser works fine with the WSE RX units but it also works fine
with many conventional transceivers - but only those that have
significant selectivity between the antenna and the mixer.
(Calibration is best done with RF amplifier off)

If the receiver has inadequate dynamic range to be calibrated with
a wideband pulse, you have to insert a bandpass filter.

> I have used a pulse generator that produces pulses about 25 ns 
> wide -- so they have significant power up to 40 MHz or so -- but 
> at a PRF of order 100 Hz the S/N in the calibration procedure is 
> marginal.
You probably tried to calibrate a receiver with a wideband input.
This is from the link above:

"By picking out about 0.5 MHz of the frequency range the single 
pulse is converted to a damped oscillation that lasts about 2 
microseconds and has a peak amplitude of about 300 millivolts 
or about 0 dBm."

It is best to use a filter with two coupled resonators.

To calibrate a receiver with really poor dynamic range, a pulser
followed by a filter, then an RF amplifier operated in class C 
will provide strong enough pulses at the desired frequency range
without adding noise.

For calibration on 2.5 MHz I can use my audio waveform generator.
It can be set to give a squarewave with sufficiently short
rise and fall times. Unfortunately it gives a rather high noise
level at it's "TTL/CMOS" output. Probably Vcc is not adequatly 
decoupled inside.

73

Leif / SM5BSZ





> 			-- 73, Joe, K1JT
> 



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