HP8640 RX LO

[W4AMV]

As I was reading the old threads on receivers. I noticed comments on the HP 8640 as an LO. I started such a receiver, utilizng direct conversion and the 8640 source as the master LO. These master oscillators are surfacing from time to time at reasonable prices. And so I was curious, has anyone taken the plunge to run these as their LO in a HB receiver and how did it work out?

Alan, W4AMV

 

[WA1GFZ]

HP8640B oscillator is very clean. I have a spare cavity that is slated to go into my homebrew RX. The cavity runs about 225 to 560 MHz. I'll hang a divide by 8 prescaler after it to make the range i need.

[W4AMV]

Yes, they are. They are also very stable, but not stable enough if the receiver has a narrow band CW filter. So, this source would have to be locked to a xtal oscillator. Or, what would be really cool, the ol huff and puff circuit, which in effect is what HP did in their original generator design.  Thanks for your comments. Just an FYI, I measured mine one afternoon, the PN was -141 dBc/Hz at 500 MHz at 20 kHz offset. So after frequency division, depending on your choice of counter, you can bring that down to -165 ! Excellent ! The stability I measured, again at 500 MHz, was less than 10 kHz / hour after an hour warm up. That is also fantastic for a free running octave tunable LO operating at 500 MHz. 

[WA1GFZ]

I've piped my 8640B into the LO of a RA6830 and ran some close in dynamic range tests. It sure beats the internal synthesizer. Yes the huff and puff would be an option. My units seem pretty stable after a half hour or so.

[KM1H]

What about the USM-323?

[W4AMV]

Sorry, not familiar with the USM-323.  ... Please fill me in on the details.

Alan, W4AMV

[W4AMV]

Ok, found it. This is the military version (rugged) 8640B. Same as the commercial 8640 without the phase lock ( huff n' puff), 1/2 digit, etc... Since the 250-520 MHz tunable oscillator is the same, sure this source is still excellent. Note, I'm not suggesting buying these generators and doing any sort of tear down for the source. However, I have seen these units tossed aside cause they were just to expensive or difficult to obtain for parts and service. That's the shame since those LO's can be saved. And even if the LO does not function, they are easy to troubleshoot and repair.

[w3jn]

AKA HP8640B opt 323

[WA1GFZ]

I have a hanger queen for parts. The switches are the real issue. Early units used nylon gears that would dry out and crack. The cavity should last for ever. The mil unit had problems because many were used them facing straight up. The grease on the mechanical parts would flow into the cavity making it act crazy. Easy fix is clean the cavity.

[The Slab Bacon]

If you have stuff getting into your cavities and driving you crazy, maybe you
should see a dentist...........................
  

[W4AMV]

Yes, I read that problem exists. All my units lie FLAT and to date, no issues. However, it is a good note and not to store the cavity oscillator vertical! Thanks for bringing it up.

[WA1GFZ]

or the special doctor Frank