Randy's Bench

A repository for projects past and present

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Notes on the MC3356 Spectrum Analyzer

I recently ran across an additional document describing the construction and use of G4PMK’s simple spectrum analyzer which is located here. Although the document is in the Spanish language you’ll find Google translate very helpful. If not I’ve included a simple, quite readable translation here. The builder did an excellent job both in construction and presentation. Here’s few photo’s of his build…

Great job!

See you soon…Randy



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Testing the MC3356 Spectrum Analyzer Part 3

First let me present some background information concerning the evolution of this MC3356-based spectrum analyzer design. Back in 1988 Radcom magazine published letters which included some experiences and tweaks from early SA builders.The majority of the user comments revolve around Helfrick’s original project reported in RFDesign Magazine from Jan.1988. Radcom magazine’s past issues are available at archives.org.
Find in the column “Technical Topics”

April 1988
July 1988
August 1988
September 1988
November 1988

This is also a good time to mention a small modification presented by the author of our current SA build, Roger Blackwell which is illustrated below:
“You can improve the upper frequency limit by improving the bypassing of the 12V side of L2 with a 1000pf chip cap or leadless disk, and by fitting a 47K series resistor in the varactor tuning voltage line close to the diode and bypassing it with an extra 1000pf leadless disc or chip cap.”


And on to business…I learned this morning that the parts I ordered are finally in the mail…Yeah! These are primarily the components for the 15 KHz bandwidth filter with a few misc. others. As the project stands the upper frequency limit I’ve seen is about 37 MHz. I tightened up the layout around the MC3356’s LO portion, along with the above stated modification and am now at 51 MHz tops. I plan to make a change to the coil (L2) in that section from it’s current 2.5t out to 3.5t. Each coil needs a bit of tweaking for proper operation dependent on layout and lead length’s (it’s VHF after all). Below is a rather poor image of an early morning setup…

Once I do some fiddling around with the coil business I’m then planning to transfer the circuit modules over to the intended double sided clad board and trying fit it all in the enclosure I’ve chosen. In the mean time I’ll think about module sizing for the filter components soon to arrive at my doorstep.
Oh and I’ve begun putting together a 3-resonator version of the 145 MHz BP filter which then will go into the HSA once I return to that project along with altering the 2nd converter frequency I mentioned earlier. Here’s a photo of the filter in process:

Here’s one more plot of my computer monitor noise displayed on our current spectrum analyzer:

Talk to you later…Randy

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Testing the MC3356 Spectrum Analyzer Part 2

I thought I would share some rather early results from our MC3356 Spectrum Analyzer project. When we left off the breadboard wiring was underway. I discovered a couple of wiring issues I had made on the sweep / video out card and made the corrections…it happens. Once the basic wiring was done I went on to check that +12V / -12V and +5V appeared in their proper places. Next, installed the IC’s and checked the MC3356 LO’s oscillation and then another trip over to the 2nd mixer to make sure it had the proper functionality – it did. I tacked on the original marker generator I had put together and used the 1 MHz square wave output as an input into the front end attenuator. Pretty much right away, without any additional board adjustments we had some functionality. Here’s a couple of the early plots taken with the 1 MHz marker generator: Looking at the 1st plot there’s quite a large magnitude spur present left of plot center. Also note the non-linearity present in the upper frequency region as well as the increased baseline noise level. It’s also apparent the input frequency span (VCO) has limited range and needs visited.

Initially adjusting the trimmer caps in the BP filter had no noticeable effect on the output. The two ceramic 10.7 MHz filters were simply inserted initially in the their sockets. I flipped the filters around one at a time, the filter on the IF output side had no noticeable effect. The filter on the IF amp input side dramatically increased the video out signal level. The 10.7 MHz ceramic filters definitely have both input and output pins defined. But, their marking’s vary considerably, and can be difficult to determine. I then re-tried adjusting the 2 trim cap’s in the BF filter and each adjustment did alter signal amplitude.
Below is a plot of the the spur mentioned above. It shows up with no input applied to the analyzer. At this point I’m unable to define it’s frequency.

Looking at this spur plot got me thinking about the HSA project a bit. I think that the issues I’ve experienced are not as much the 145 MHz BP filter itself but the choice of the 2nd mixer oscillator frequency (set at 145 MHz). Adjusting the LO to a frequency 10.7 MHz above or below the Bandpass center frequency or possibly other frequency’s such as 8 MHz with xtal filtering. I would be confident this would alleviate the issues observed when zooming in on a particular signal.
But back to current SA, Here’s the current bench setup arrangement:

And a final plot using the 1 MHz marker as input:

With that I’ll run thru a list of current issues I’ve come across. After all, what’s a project without issue’s anyway?

A. The Zero span doesn’t work as intended. Opening up the VCO sweep line should give manual tuning control using the center frequency control pot. But, does not. I matched the VCO ramp voltage range to that produced by center freq pot and looks good. Also the pot acts a little wonkie as well watching the video out. If I rotate the pot back & forth rapidly I can see the 1 MHz signals on the screen but not otherwise. Well this needs some work… Also I plan to put the fine center frequency pot back in place. I removed in the published schematic but retained it on the sweep card and find it useful.

B. As mentioned the varactor frequency span isn’t correct. 145 MHz to 235 MHz is the stated desirable range. With an open air coil in place without a tuneable ferrite core some coil / cap tweaking is called for here.

C. Not an issue but with the initial success experienced so for with the bandpass filter I’ll give a triple resonator version a go with the final design and layout.

Electric dreams…Randy

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Testing the MC3356 Spectrum Analyzer Part 1

Were are we? I’ve added the modules completed so far to a single-sided copper test bed. The 2nd mixer (SA602/612) LO has been calibrated to oscillate at approx. 134.3 MHz plus or minus but that’s about it at this point. I’m still missing the 15 KHz filter portion of the SA due to a rather slooooow parts vendor but for now, not necessary.

Here’s a few pix’s of the build so far:

Also I put together a small panel that holds the coax connectors for system I/O:

The 145 MHz bandpass filter needs some tweaking from that of the original design of a 144 MHz up a MHz by trimmer cap adjustment and spreading out the resonator coils a bit. I’m thinking about how to terminate the bandpass filter I/O. Not quite ready for this yet but likely will purchase some SMA bulkhead connectors and a couple of short SMA jumpers…Seems reasonable. For the current build I’m going to use (2) 280 KHz 10.7 MHz ceramic resonators for IF filtering since… well, that’s what I have in the bins. These filters I salvaged from some older portable AM/FM radios I usually pick from various thrift stores around town. Their cheap and contain lots of useful parts. Additionally breadboarded a 20dB 50 ohm in/out attenuator for initial board testing. Always protect the input…

Once the board wiring is substantially completed I’ll 1st start by checking the 12V and 5V connections before inserting the ICs (good idea).
Then it’s time to check the frequency span of the 1st mixer (part of the MC3356 chip). Here I’ll take a look at the VCO sweep voltage range and note those reading, place the the Sweep card in zero span and adjust the center frequency control within those boundaries and see what we get for frequency swing.

Oh and I finally get to use the turns counter knob I picked up many years ago for the center frequency pot..Yeah!

But first I’ve a bit of wiring…

Build something Analog!




RF Beginnings for the MC3356 Spectrum Analyzer

With the sweep generator and video amplifier in good shape (Note: I replaced the video amp IC with a CA3140 chip), I’ve moved on to construct a few of the RF section modules while I patiently wait (not really) for additional parts to arrive. I’ve found the MC3356 ICs readily available on Ebay for a reasonable cost and had 2 DIP chips within a week. Below is my adaptation of Roger Blackwell’s, G4PMK, original RF section schematic with article errors corrected (that I’ve found so far – it’s a work in progress).


Here’s a higher resolution version of the above RF schematic

I’m currently using open air coils for the 1st and 2nd mixer circuits which are a pain to adjust with the expanding/compressing necessary to get on frequency. Once the slug-tuned coils forms arrive I’ll be swapping these out. So far I’ve put together the IF amplifier, the 2nd mixer, the 5V regulator, and the 1st mixer / log amp board (in progress) as separate modules. This will give some flexibility as to placement on the main RF card.

Here’s the IF module as it stands

5Vdc 78L05 regulator board

And the 2nd NE602 Mixer

Upon Initial powerup of the 2nd mixer the LO refused to start up. No surprise here as I’ve seen this before with the HSA analyzer. Running the LO in excess of 100 MHz can be problematic for reliable operation. The trick is to add (parallel) the existing internal LO transistor’s emitter resistor (25K) with a 22-27K resistor to circuit return. Basically add a resistor from the NE602 pin 7 to return. This increases the DC bias current available for the oscillator and helps it to start up.
I took a preliminary look after this step and it seems to work more or less. The LO is in the very rough ballpark of the 134.3 MHz we are looking for but seems to have a lower frequency modulation effect going on, and of course, the frequency adjustment is difficult at best. I’ll rework this module once I get a slug tuned version of the LO coil in place and dump the trimmer cap as well.

I ‘ve taken an early look at how the enclosure placements might be down the road. At the rear right of the box is the dual polarity linear supply I intend to use. I picked up several of these years ago and are nice and quiet. I believe the supply supports 240mA total. Should do the job…
Was also thinking that with the addition of a detector and audio output circuitry the SA would serve, using the zero span switch and center frequency controls, as a useful AM shortwave bands receiver…why not

I’ve decided on a 145 MHz bandpass filter design after a bit more education on the subject. This comes from a Wes Hayward article from December’s 1991 QST entitled “The double-tuned circuit: An Experimenter’s tutorial”. The PDF is simple to find either at QST’s archives or online. I thought this would be a solid, well described filter to get started before tackling a triple resonator variety. This guy will as well retrofit into the HSA board.
As you were…Randy

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Sweep Generator for the MC3356 Spectrum Analyzer

I‘ve made some progress with the construction and preliminary testing of the sweep generator and video amplifier for the new MC3356 SA project.

Below is my adaptation of Roger Blackwell’s, G4PMK, original sweep generator schematic…

Here’s a higher resolution image of the Sweep schematic

I’ve added a dual opamp to the VCO output section to accommodate an inverting unity gain amplifier which better suits how I wish to sweep the MV-209 varactor diode in the 1st mixer oscillator. I used a 1458 chip because I had some in the bins but could be just what you’ve got…LM358, another 741 or other. I removed the fine center frequency pot as is unnecessary in this design (IMHO). Additionally I added a zero span switch which will stop the vco from being swept and allow control using the center frequency adjustment. I removed IC7,R22 and R23 and jumpered that section out as shown to better accommodate my scope collections X-axis sweep direction. Let’s see what else, I had some difficulty getting the 555 timer to reliably oscillate using the .47uFd timing cap as shown and went with a 1uF for now. That works well now and fits with Helfrick’s original QST design article. Also replacing the 555 timer chip with the lower power CMOS version caused issues. For now I’m using the standard issue NE555 but will revisit this down the road. I may also upgrade the video output opamp as well with a lower noise, higher input impedance chip. Plus, I may add some adjustable gain to IC9-B that may be switched in and out…We’ll see

One other item, I added an additional VCO output for the project’s front panel just in case I look at doing a tracking generator for this…you never know.

Plot 1: X-Axis sweep output taken @ RV2

Plot 2: X-axis sweep vs.Blanking pulse @ IC11 pin 2

Plot 3: VCO drive output taken @ IC9-B pin 7

This all looks like a reasonable start for this functional module and will tweek as the need presents itself.

I’ve done some more investigating on the subject of Marker generators and came across one in an issue of Ham Radio Magazine (July 1988) which looks quite promising. You’ll find a copy of that article here. (A simple low-cost comb generator frequency calibrator by Larry Martin)

Additionally I’ve (tried) to get some better understanding regarding double and triple tuned helical bandpass filters and, after some reading, realized I did a rather poor job in implementing the 145 MHz BP filter in the HSA project. It’s about recognizing the important elements in the design and construction aspects of these filter’s, especially in resonator coupling and shielding. Since a similar filter is employed in the MC3356 Spectrum Analyzer, it’s high time for it! Like Edison said “I have not failed. I’ve just found 10,000 ways that won’t work

Carry on…Randy

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An Old Issue and a New Beginning

I thought that while I’m considering how to handle the issues I’ve experienced with the HSA 145 MHz bandpass filter that I would take a look at an alternative spectrum analyzer.
With that I’m currently looking at two designs. The 1st appeared in RF Design magazine back in January 1988 and authored by Albert Helfrick. His work revolves around a now obsolete (but still available) Motorola MC3356 FSK receiver IC. This chip includes, besides other features, a mixer, oscillator, IF amplifier and logarithmic output capabilities in a 20 pin package. Helfrick’s design may be found here. The 2nd design was a continuation of the above mentioned SA in that it added additional features including a dual frequency marker generator, a revamped sweep generator, multiple resolution BW filtering and improved input attenuation. Look here for Roger Blackwell’s Radcom article from November 1989…

I’ll begin with the 10 MHz/1MHz marker generator module and go from there. I’m waiting on some MC3356 ICs to show up, no hurry with that. (They showed up today…)



10 MHz Output

1 MHz Output

The plots above were captured with the individual outputs separated i.e. the two 4.7pF caps (C43 & C44) not connected together. When the 1 MHz output is enabled, by design, the 10 MHz oscillator is additionally enabled by way of D6 (+12V – diode drop), the outputs interfere with each other. Not sure what the original idea was here…for now that’s OK and I’ve moved on to the sweep generator portion of the project. I’ll revisit this problem with likely an alternate design or different switching arrangement.

Next time…Randy