RE: [linrad] Calibrating analog hardware

[Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxx>]

Hi Roger and all,

> I have done the calibrations both at the IF frequency and at the RF
> frequency.  I haven't done any in probably a year, and so I am 
> relying on my
> aging memory and some notes here.
A year is a long time and I think you will find that the image
suppression is not as good as it was before.....

For the spur to stay at -70 dB or better, components values have to stay 
constant within something like 200 ppm. If the spur does not bother you, 
just don't care:-)

The frequency response calibration that you make with a pulse generator 
is best obtained at the RX2500 input if you have a pulse generator
with 50 ohm output impedance. At a low frequency like 2.5 MHz you can use
long pulses and get a lot of pulse energy in each pulse. A 6dB attenuator
will bring the low impedance of 5V logic ICs close enough to 50 ohms.

The input stage of RX2500 uses noise free feedback so there is a small
effect on filter characteristics from a mismatch at the RX2500 input.
If calibration is done with a low impedance pulser at the RX2500 input,
the error in the noise floor flatness is about 0.2 dB. A 3 dB 
attenuator reduces the error to below 0.1 dB and a 6dB attenuator makes
it invisible.

You can also connect the pulse generator to the RX10700 input. This way
the RX2500 will see the correct input impedance but you will add 
the frequency response of the RX10700 into the calibration. 
The RX10700 if very flat, but it may differ a little depending on what 
frequency range you selected.

When using the WSE converters there is no reason to calibrate at
earlier stages - but you can do it if the pulse generator is good enough.

73

Leif / SM5BSZ


 


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