Signal Generator

[Steve Harpole, DDS.]

Any suggestions or recommendations for a Sig Generator used for applications such as tracing as described in Appendix E of manual or at elecraft's site.  I am aware of the simple oscillator with 10MHZ crystal that is described after the Sig tracing section of appendix.   I have found that the signal generated for my scope 199C by fluke needs to be in the order of 20millivolts for the 10 to 1 probes.  Any suggestions or "redirection" is appreciated..hi..hi..

Steve
KQ4UB
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[Don Wilhelm-3]

Steve,

If your 'scope will indicate down to 20 mv (I assume that is peak to peak),
you should be quite able to use the oscillator shown in the K2 manual - the
lowest expected voltage indicated in the manual is 0.27 Vrms at the 2nd
Crystal Filter Output and that is 756 millivolts peak to peak.  In fact,
with a 'scope that sensitive, you could use a signal source of only
one-tenth the 140 mVrms value stated and achieve similar results.

73,
Don W3FPR

----- Original Message -----

Any suggestions or recommendations for a Sig Generator used for applications
such as tracing as described in Appendix E of manual or at elecraft's site.
I am aware of the simple oscillator with 10MHZ crystal that is described
after the Sig tracing section of appendix.   I have found that the signal
generated for my scope 199C by fluke needs to be in the order of
20millivolts for the 10 to 1 probes.  Any suggestions or "redirection" is
appreciated..hi..hi..



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[G3VVT]

 
If you are checking the gain of the whole K2 RF and IF strip less the  
modification of the IF gain by the effects of AGC, yes, it is possible to use a  
lower output from the signal generator.
 
However if you need to go into troubleshooting in greater depth, the 270mV  /
756mV pk-pk found at the 2nd crystal filter output (product detector input)
is  far from the lowest signal that will need to be measured. The input to the
to  the RX mixer Z6 with the preamp and attenuator switched off is listed as
only  77mV / 215mV pk-pk after the nominal 6dB voltage loss going through the
low  pass/bandpass filters when 140mV / 392mV pk-pk is inserted at the antenna  
jack.
 
Whilst this will seem a relatively high level there are several pitfalls to  
beware of.
 
Firstly, the oscilloscope will probably be using a X10 high impedance probe  
which would only give 39.2mV pk-pk to the input of the oscilloscope.
 
Secondly, most of the older oscilloscopes had a maximum sensitivity of 10mV  
pk-pk/cm and with this there could be a further caveat that the bandwidth of
the  oscilloscope was reduced on the lowest range. The common 5MHz bandwidth TV
 service oscilloscopes of the past came down to only a bandwidth of only 1MHz
or  so when on the 10mV range. This is not much use when trying to measure RF
levels  even on 80m.
 
The high signal generator output level advocated by Elecraft in their K2  
troubleshooting info though needed for the use of their supplied RF probe/DVM,  
may be of significance in doing the same measurement with some earlier  
oscilloscopes. Modern high bandwidth oscilloscopes with a  maximum sensitivity of 1mV
pk-pk/cm would fare much better on this score.  You need to check out what
your particular oscilloscope specifications are and  that it can fulfil the task
before jumping in with both feet to do the  measuring.
 
I approach the troubleshooting of the RX front end and similar measuring  
tasks by using an RF millivoltmeter with a high impedance probe. Was able  to
acquire a Racal Dana 9301A some years back and that will go down to 1mV FSD  at
up to 2GHz. This means that I can use much lower levels of RF injection into  
the antenna jack to do measuring in the RX RF section. Found it very useful to  
repair a JRC RX whose front end had been very extensively damaged by a  
nearby lightning strike. The problem is that modern RF millivoltmeters can be as  
costly to buy as a good signal generator unless an earlier tube  type can be
located. There used to be several types available from  manufacturers like Heath
that would measure RF quite successfully and now are  available at very low
prices.
 
Bob, G3VVT

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