[time-nuts] HP 105B Modification

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 8 04:33:08 UTC 2009


Since the T13-1 has a centre tapped secondary there are 2 useful 
amplifier configurations that can be used.
One has a nominal gain of about 5dB whereas the other has a gain of 
around -1dB (can still be useful since the amplifier input impedance at 
10Mhz is about 1k when the 51 ohm input shunt resistor is removed).
LTSpice schematic files for these are attached.
The emitter resistor values shown are a good starting point however some 
adjustment to compensate for transformer parasitics may be useful to 
improve the output match.


Bruce

Ed Palmer wrote:
> I was wondering about that transformer.  Thanks for the cross-reference.
>
> Ed
>
> John Miles wrote:
>>> Ed
>>>
>>> If it helps I can send you some LTSpice schematics so that you can
>>> simulate the circuit for yourself.
>>> The breadboards behave as predicted by the simulations at 10MHz.
>>>
>>> John Miles has done some preliminary phase noise measurements on his
>>> version.
>>>
>>> The transformers are wound on binocular ferrite cores.
>>> I used some 14mm (long) cores intended for 40MHz to 220MHz (I had some)
>>> operation in my breadboard which works well at 5MHz and 10Mhz.
>>
>> You can also use an off-the-shelf Mini-Circuits transformer for 
>> low-power
>> applications.  The T13-1 was the one I tried.  I'll stick some of the 
>> plots
>> up on the web later tonight if possible.
>>
>> Residual PN is very good with this circuit, with a broadband floor 
>> near -170
>> dBc/Hz at 10 MHz, and no excess 1/f noise relative to a passive mixer.
>>
>> -- john, KE5FX
>>
>
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