The HP 3586 Selective Level Meter (SLM) is a great piece of test equipment. It is a super HF receiver with lots of measurement bells and wistles. One drawback of this equipment though is the fact that one of the mixing stage LOs is not derived from the on-board or externally fed 10MHz reference. The last stage, the product detector has its BFOs (LSB, USB) free running using regular quartz crystals. Unfortunately these drift. The drift is insignificant and non-perceivable if you only listen to the SSB audio. But if you perform long term tone frequency measurements, this is a show stopper.

The plot below shows how much SSB BFO drift I detected using a stable carrier fed into the RF input of the SLM. The audio tone produced by the carrier was measured on a PC sound card using Spectrum Lab software. This drift was caused by day-night-day variations in room temperature. This was induced by a variation of around 2 degrees Celcius. You can actually see the house heating system's behavior, with a major night time reduction of temperature, a few heating cycles overnight and a temperature recovery phase in the morning. Note that the sound card drift was also measured, but it can be ruled out as a major contributor in this plot since it drifts in the opposite direction as a function of temperature.

IMPLEMENTATION

The HP 3586 SLM uses two or three SSB LO frequencies internally, depending on the model. They are the USB, LSB and Calibration Mode frequencies. These frequencies vary as a function of the unit model and the equipped options. Here is a table extracted from the Service manual that summarizes the various configurations:

HP 3586 Single Sideband (SSB) L0 Frequencies.
Model	Opt. 003	Cal Active	Bandwidth	Sideband	Crystal	Crystal Freq.	SSB L0 Freq.
3586A/B	YES	NO	3100Hz	USB	A22Y102	1.7475MHz	17.475kHz
3586A/B	YES	NO	3100Hz	LSB	A22Y101	1.3775MHz	13.775kHz
3586A	YES	YES	3100Hz	-	A7OY1	1.6425MHz	16.425kHz
3586B	YES	YES	3100Hz	-	A7OY1	1.6625MHz	16.625kHz
3586A	NO	NO	1740Hz	USB	A22Y102	1.6975MHz	16.975kHz
3586A	NO	NO	1740Hz	LSB	A22Y101	1.4275MHz	14.275kHz
3586B	NO	NO	2000Hz	USB	A22Y102	1.7125MHz	17.125kHz
3586B	NO	NO	2000Hz	LSB	A22Y101	1.4125MHz	14.125kHz
3586C	-	NO	3100Hz	USB	A22Y102	1.7475MHz	17.475kHz
3586C	-	NO	3100Hz	LSB	A22Y101	1.3775MHz	13.775kHz

So the proposed solution has to be reconfigurable on-the-fly, as the SSB mode is selected by the user via the front panel pushbuttons and when the Calibration state is invoked by the unit's firmware. It also has to be flexible enough to accomodate the various SLM versions.

The Analog Devices AD5932 is a great little chip. It is a DDS-based Programmable Frequency Waveform Generator fitted inside a TSSOP 16-pin package. This chip has a frequency divider with 2²⁴ steps. This means you can put out a clock with a division resolution of 2²⁴ steps applied to the input reference clock. What you also need is an accompanying chip to load the AD5932 with the right configuration at power up or when the SLM SSB mode is changed. An 8-pin PIC12F683 is perfectly suited for this task.

Except for the 10MHz clock reference, all input and output signals to the proposed circuit are 5V-TTL logic, so this is a simple interface to the HP 3586, with a 5 VDC supply applied to both the AD5932 and the PIC micro. The 10MHz reference signal needs a bit of waveform amplitude and offset scaling to make it a TTL-compatible clock; that can be perfromed by a single NPN transistor. Both chips and the transistor fit on a small add-on PCB that connects to various points within the SLM.

Here is a block diagram that shows how this solution is implemented inside the HP 3586 SLM.


Here is a schematic diagram of the proposed add-on circuit. Click on the figure to see it in full size.


Inside the HP 3586 SLM, almost all connections are made to module A22, one of the plugins inside the unit. Here is a schematic page extracted from the HP 3586 Service Manual that shows the required hookups of the above circuit to module A22. The annotations in red and pink are all there is to it as far as the SLM itself. Click on the figure to see it in full size.


The diagram above also shows the A22 board layout on the left hand side, which will be helpful to perform these mods.

I have elected to extract the 10MHz reference signal that drives the DDS chip directly from the A40 plugin (the reference plugin) 10MHz output port normally connected to the rear panel. In my case I don't need the 10MHz output signal since my SLM does not have the OCXO option.

There are other locations within the SLM where a 10MHz reference could be tapped off. These other points all use the PECL logic technology. Consequently, a re-configuration of the 10MHz transistor circuit would be required. Though I have not tried it, the following circuit would be a better alternative when dealing with PECL logic swing since it has a smaller swing. This is essentially a gain stage, with a gain of -5. Consider this circuit as just a starting point; you may have to tweak the resistor values to center the DC bias to the PN junction operating range.

I would appreciate hearing from those who try the PECL avenue. I could update this page with the information.

Note that this upgrade to the SLM has no impact to the built-in frequency and amplitude measurement capabilities and specifications. This mod only affects the product detector and the resulting SSB audio.

PERFORMING THE A22 PLUGIN PREPARATION WIRING

Here are two pictures showing the wiring work required in preparation for receiving the add-on board on the A22 plugin. Click on the figure to see it in full size.


The following are a few instructions and notes that should be taken into account when performing the mods on the HP 3586 A22 plugin card.

• One the picture above, please disregard the blue wire. This was added for previous experimentation of this circuit and is not required anymore.
  
• Working form the top of the 3586 unit, cover removed, you must remove the eight screws on the A22 faceplate, then pull out the plugin using the two knobs.
• All added wires but one are soldered to the component side of A22.
• The Axx row of card edge fingers is on the component side of the plugin PCB, the Bxx row is on the solder side.
• On the solder side of the A22 plugin, perform a cut on the trace that connects to card edge finger B2. Use a sharp exacto knife. See solder side picture above (upper-left corner).
  
• Again seen on the solder side picture above, the purple wire is the MSBOUT signal and it must be soldered to the solder side of the PCB, and routed to the component side through the nearby hole. That wire may be connected to card edge finger B2 or to the trace downstream from the trace cut and toward card edge finger B2. If connecting to the finger, make sure you solder the wire to the very edge of the finger, keeping in mind that the fingers get inserted into the A22 motherboard edge connector. I used kapton tape to keep the liquid solder from envading the entire finger and contaminate the gold plating. Refer to the picture above. If connecting to the trace, you will have to scrape off the solger resist to expose some copper.
  
• The Cal Mode signal, shown as a green wire on the picture above, is only used on the Option-003-equipped SLMs. Disregard otherwise and leave unconnected on the PIC.
  
• This Cal Mode signal can be connected to the trace via hole shown in the picture. Follow the trace from the card edge finger B15 (solder side) to locate the via hole where to solder to on the component side.

Note: It appears that there exists a second version of the A22 board. There are major omponent differences and the board does not look the same when comparing to the pictures above. Though I have not seen a board of that other version, there is another version of HP 3586 circuit schematics that details it. If you happen to have one of these different A22 boards, please communicate to me the changes you had to make to adapt my mod to the other A22 board version. I will update this web page. Thanks.

ASSEMBLING AND MOUNTING THE ADD-ON BOARD ON A22

For this stage, there is more than one option; mine is one suggestion, but you may adapt it based on the availability of the PCB material or your junk box.

My add-on board is made of two PCBs, a prototype PCB and the TSSOP chip carrier PCB. The former is a piece of plated-through prototype PCB. The latter is a ready-made adapter board I purchased on eBay for a few dollars. Searching for "SSOP DIP adapter" on eBay will yield several solutions. Hobby electronics components websites are also a good place to look for these.

Here are a few pictures showing how I put the add-on board together.


My TSSOP carrier PCB has extra pads to install additional discrete components. With a bit of planning, I could place all surface-mounted resistors, capacitors and the transistor on the TSSOP carrier PCB.

• I used parts of a 0.1 inch header strip to mount/connect the TSSOP carrier PCB to the prototype PCB. I suggest you first solder the AD5932 chip onto the carrier PCB before soldering the header pins and the other components. This gives access from all angles for the soldering procedure.
  
• Soldering the AD5932 chip to the carrier PCB is not an easy task and it requires skill, steady hands and the right soldering iron, using the finest tip there is. Hot air soldering is also possible if the pads are pre-tinned or if using solder paste. In all cases, use liquid rosin flux to help the soldering process. A good microscope or stereoscope is mandatory; a magnifying lamp is not enough unfortunately. If you have not done this in the past, ask a friend who has done it before for help. It may also be the right time for you to consider upgrading your soldering/re-work tools...
• I used (AWG-30 size) wire-wrap wire for the interconnects between the chips. I strip both ends of their insulation and solder the wires to the pins.
• On the A22 plugin, I used the remaining corner mounting hole to install a standoff post. I mounted the add-on board to the standoff. It keeps it in a fixed location, away from the A22 components. Make sure that the final assembly will fit back into the A22 compartment without causing any short circuit. Notice that there are a few components mounted on the motherboard inside the A22 compartment. The positioninng and size of the final add-on board has to clear these components once the A22 plugin is fully inserted.
  
• Make sure there is not too much air flow restriction as a result of the add-on board presence. The cooling air is forced from the bottom of the SLM into the A22 compartment through a few holes in the motherboard.

Here is a picture of the completed modification to the A22 plugin.

CONNECTING UP THE 10MHz REFERENCE

• Disconnect the SMB coaxial cable from port J3 on the A40 plugin faceplate. Connect a new coaxial cable with a SMB connector at one end to J3.
• Route the new cable towards the A22 plugin.
• Pass the cable through one of the ventilation holes on the A22 faceplate.
• Solder the coax center conductor to the transistor circuit on the add-on board. Leave the shield unconnected at that end. Make sure the shield will not come in contact with anything. I used heat shrink tubing to cover the exposed shield.

The completed coaxial connection is shown on the picture below.

THE PIC MICRO FIRMWARE

I wrote the source code in BoostC, an ANSI C compatible language. This compiles within the SourceBoost IDE, a free programming environment. The Hex files required for programming the PIC12F683 for the various HP 3586 versions are also provided below.

HP 3586 SSB L0 Upgrade - PIC12F683 Firmware
Model	Opt. 003	Source Code	Hex file
3586A	YES	HP3586_AD5932 Ctl.c	HP3586_A_Opt.003.hex
3586B	YES		HP3586_B_Opt.003.hex
3586A	NO		HP3586_A.hex
3586B	NO		HP3586_B.hex
3586C	-		HP3586_C.hex

RESULTS AND PERFORMANCE

Operation
After the modifications are performed and everything is assembled back together, power up the unit. The SLM behavior should be exactly the same as before when it performs its power up self-tests and when you listen to radio signals. Use your best frequency measurement technique to verify that the unit is right on expected frequency and that it does not drift as a function of temperature.

Again, this upgrade to the SLM has no impact to the built in frequency and amplitude measurement capabilities. This mod only affects the product detector, so the speaker audio and rear jack demodulated audio.

Stability
the main intent of this mod is to stabilize the SLM. The following plot shows the drift of my HP 3586B before and after the modification was applied. The heat up technique involves blowing hot air into the rear air inlet (fan intake) for 30 seconds, using a heat gun (paint stripper). The results speak for themselves...


Accuracy
The table below shows the delta in Hz - Targeted vs.Resulting - after the modification is performed to the SLM. This is the closest that the AD5932 can produce to the intended frequencies with a 10MHz reference as the MCLK signal. As can be seen, the delta is less than 0.3 Hz from any targeted LO frequency. This same absolute delta in Hertz is applied to any detected RF signal. So a 10.000MHz received radio signal would have a detected audio frequency error of better than 3x10^-8 or 0.03 ppm, assuming a LO delta of better than 0.3 Hz. Actual error can be computed by doing Error_Hz / RxFreq_Hz, using the delta numbers below. Since these are known fixed deltas in frequency, this can be used to compute the exact frequency of any audio tone or received RF carrier.

HP 3586 Single Sideband (SSB) L0 Frequencies: Targeted and Resulting
Model	Opt. 003	Cal Active	Sideband	Targeted L0 Freq	Resulting LO Freq	Freq Delta
3586A/B	YES	NO	USB	17475Hz	17474.88976Hz	-0.11024Hz
3586A/B	YES	NO	LSB	13775Hz	13775.22945Hz	0.22945Hz
3586A	YES	YES	-	16425Hz	16425.25196Hz	0.25196Hz
3586B	YES	YES	-	16625Hz	16624.92752Hz	-0.07248Hz
3586A	NO	NO	USB	16975Hz	16974.80679Hz	-0.19321Hz
3586A	NO	NO	LSB	14275Hz	14274.71638Hz	-0.28362Hz
3586B	NO	NO	USB	17125Hz	17125.01049Hz	0.01049Hz
3586B	NO	NO	LSB	14125Hz	14125.10872Hz	0.10872Hz
3586C	-	NO	USB	17475Hz	17474.88976Hz	-0.11024Hz
3586C	-	NO	LSB	13775Hz	13775.22945Hz	0.22945Hz