>Rich, you say;
>
>>>....... My experience with 723s (backed up by a fair
>>>number of engineers who've had similar problems) is that they are very
>>>prone to RF pickup.
>
>>This is a non-issue. The fix is 2, 100k, 1/4w resistors at the 2 input
>>pins.
>
>Don't agree. For RF fields picked up on the chip ( and we've done some
>measurements on that) the 100K resistors don't do anything. In fact,
>they increase the impedance that is seen looking out. Don't have the 723
>data sheet to hand: what's the input bias current and the input bias
>current offset? 100K could be a bit high to offer best performance .
>
RE: RF fields picked up on the chip: This would have to something in
the GHz range. I thought we were talking about a HF amplifier, Peter.
- Here's how I made the 723 screen regulator resistant to RF. Taking the
advice of Joel, N6AMG (rip), I held a 2w, 2m hand-held near the 723.
After some trials and errors, I arrived at the aforementioned anti-RF
techniques.
Rich's design philosophy: There is ALWAYS a performance trade-off to
making a circuit more stable.
- For instance, the c.100k ohm input resistors undoubtedly decrease the
regulation ability of the 723, however, after the 723 was made
RF-resistant, I performed a load regulation test. With the output set
for 1100v, a 250mA load was placed on the screen supply. The output v
under load was 1100v. (measurement taken with John Fluke 8022A DMM)
Sure, a performance drop would no doubt have been detected if a 4 1/2
digit DMM had been used instead of a 3 1/2 digit DMM--but for a AB1
screen supply, would it matter, Peter?
- For instance #2: The unavoidable trade-off with the use of high
VHF-Rp/low VHF-Q parasitic suppressors is a roughly 1% decrease in 29MHz
power output. Is such a reduction in output just below the VHF range
worth the price? I have a collection of kaput gold-sputtered 8877s and
grid-fil. shorted 3-500Zs which suggest to me that it is.
- Rich -
R. L. Measures, 805-386-3734, AG6K
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