5065A Synthesizer replacement - Preset Decade Counters? - Page 1
Forum section: electronics / general

author: Uky
author_rank: Regular Contributor
timestamp: March 03, 2025, 10:45:12 am
content: Forum,I am considering replacing the DTL version of the ~ 5MHz presettable synthesizer in the 5065A with amore modern (equivalent) version of the unit that uses what was then more modern integrated circuits.It will be no problem to update the RF-parts, but for the digital (preset) functions, it is another story.The four counters "1820-0332" is Signetics N8280 which is SN74176 which could be replaced (?) by 74LS196.All of which are obsoleted. The circuitry in the HP manual is more or less that of the signetics datasheetfor the 8280 (I have that databook - Never throw away anything...) and there are a not that manyother presettable binary counters which could perhaps/possibly fill the same function. There is alsothe question of synchronous vs asynchronous functions.Suggestions anyone?Another part that attention is the on-shot 1820-0313 (TO-5 metal can) But I think that that would be lessproblematic to find a high-speed replacement.I have two old DTL units so there is no panic but having replaced almost everything else butthe Rb cavity it would be intersting to learn what can be done about the synthesizer.Comments invited.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 03, 2025, 09:15:51 pm
content: Quote from: Uky on March 03, 2025, 10:45:12 amYou can still buy SN74196N in DIP.How is this preset and what Fin and Fout are needed?Your heading says decade, and the data says decade, so the binary may be a typo?The modern 'similar' part would be a CMOS 74HC160, but notice there are subtle differences.176/196 says falling edge clk, 160 is rising edge clock176/196 are async load, 160 is sync loadThere is also 74HC192, Up/Down BCD with separate clocks. This has async load, but does not chain as easily.If you just want to divide by some fixed number, there is also 74HC40103 that can divide 2..256If the setting is fixed and just division, you can also use any binary counter as /N, you just move the preset value to get desired clocks to overflow.eg the SN74LV161A/SN74LV163A will be around the fastest preset programmable dividers.There is also HC390, which can do up to /100

author: Uky
author_rank: Regular Contributor
timestamp: March 03, 2025, 10:25:54 pm
content: Input frequency is 5 MHz.The Signetics databook dated 1974 describes the N8280 as a Decade counterthat are 4-bit systems and they can be used as Counter or Storage element.The 8280 can be connected in the BCD counting mode, in a divide-by-two and divide-by-fiveor in the Bi-Quinary mode.And I missed the text a bit down, "Counting operation is performed on the fallingedge of the input clock pulse".The synteszer has 4 thumb wheel switches with position 0...9 that presets the countersusing the 4 preset inputs coded as BCD.As has been mentioned in another thread, the posibility to replace the synthesizer circuitrywith a modern uP syntesizer is perhaps a better way. Once the division ratio has been setand verified it is not changed. The required granularity at around 5.315 MHz is 0.1 Hz.Using a phase locked VHF syntesizer and dividing the RF down to 5 MHz is an option.Maybe I will consider that option instead.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 04, 2025, 12:43:05 am
content: Quote from: Uky on March 03, 2025, 10:25:54 pmSo that can preload with 0000 to 9999 and clocks from 5MHz?Quote from: Uky on March 03, 2025, 10:25:54 pmThat's a bit confusing, is there more here than a simple divider?4 thumb wheel switches will be nowhere near 0.1Hz granularity in 5.315MHz - you are asking for 5.3150001MHz on the next step?Perhaps you meant 5MHz reference frequency and ~5MHz VCO, where you divide the 5MHz reference to 1Khz and use the 4 switches to set 1kHz steps allowing 5.315MhzOptions are ~5MHz VCO & /5315 to 1kHz PFD.or you could use ~20MHz VCO and /5315 to 4kHz PFD, with VCO/4 as output.or you could use ~40MHz VCO and /5315 to 8kHz PFD, with VCO/8 as output.etc

author: edpalmer42
author_rank: Super Contributor
timestamp: March 04, 2025, 01:50:37 am
content: I don't know if it will help, but I've attached the table that shows thumbwheel settings vs. frequency. I always considered this to be very confusing and just wrote it off as being due to the old technology that was available at the time.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 04, 2025, 05:43:20 pm
content: Yes the thumbwheel settings can be confusing. The output of the synthesizer is the "m" harmonic of the 5MHz divide by N. This leads to places where a one digit change to the thumbwheel causes a 225Hz change and places where a 1590 change causes only a 38 Hz change. Also many thumbwheel setting wont work as the harmonic lies outside the capture range of the VCXO. just stick to the thumbwheel settings in the manual!I have replaces the A1 synthesizer module with a couple different DDS modules and circuits. Works fineOne other thing to keep in mind, the current flow of the A1 module is routed (along with a couple others) through the TED (thermo-electric device). This is used to regulate the amount of Rubidium in the cell. If care is not taken to keep the "new" modules current close to that of the original A1 the Rubidium level in the cell may take a couple weeks to stabilize at the new temperature.Cheers,Corby