MC1496 Double Balanced Mixer Project Page

 

Accidents happen and this project page is accidental. I participate in a regular Podcast with N2CQR, Bill. In SolderSmoke Podcast #224 Bill was reviewing the mailbag and lo and behold he shared an email from Paul, VK3HN about a new transceiver he created.

VK3HN attended a Radio Rally (read hamfest) where he found an assembly that was part of a transmitter project as described in Solid State Design for the Radio Amateur. (pp203-205). This assembly was an IF Module and Transmit Mixer Stage. Paul added other bits and pieces which ultimately resulted in a multi band transceiver that would be used for SOTA and other remote operations. Appropriately he named it The Prowler.

 

[Please Note that this DCR receiver project has actually been replicated by two very dear friends in the UK and they have developed a voluminous package of details about their builds. They have fabricated the DCR; but more importantly have implemented the circuitry differently than I did. So you will get a rare opportunity to see the same "efficacy effect" using different rodent traps. That is what I love about ham radio -- always the experimenter's platform. See the links below as a starting place .]

 

www.fishpool.org.uk/dcrx.htm

 

Builders Documentation from G4WIF and G8INE is on the link above.

 

You Tube MC1496 DCR

I personally have built many of the projects in SSDRA; but never that one. So I was off and running. Firstly, I looked at the Motorola Data sheets and thought why not try my hand at building a Direct Conversion Receiver. I had a stash of MC1496's going way back to when a I built a transceiver designed by K1BQT using the MC1496 for various modules.

Don't be under whelmed as this is the main board showing the MC1496 and the Audio Amplifier Stage. A note here for those who have nothing better to do than to pick things apart. This is a shot of the first version that had an LM-380N and that now has been changed to an LM-3856. You simply did not need all that output and the schematic later on shows the LM-386

 

Motorola Application Note ~ MC1496

For my Direct Conversion Receiver, I simply followed the application note schematic for the Product Detector. Other bits included a 2N3906 PNP RF Amplifier Stage, a Band Pass Filter and a IC audio amp. For the Local Oscillator, I programmed an Arduino Uno R3 and of course the Si5351.

All projects have to start with a Block Diagram and is shown below. Since VK3HN named his rig "The Prowler" , sort of like an Alley Cat prowling around looking for female companionship and or a midnight snack, I hit upon calling it "The Cat-Around". The hidden meaning is always around looking for the Double F (Females and Food). Imagine taking the DCR to a park bench and firing it up --it will be like a Chick Magnet!

So let us look at the other block pieces to this project. The first bit is the Arduino / Si5351 LO. I want to get this behind us so we can concentrate on the rest of the block modules. The Direct Conversion Receiver operates with a ZERO IF which means the signal you inject is more or less at the operating frequency. But there is a hitch as you will receive the same signal at two places on the dial. Think of it as receiving both upper and lower sideband at the same time. It is not a single sideband. The diagram below explains the why.

 

An off the air signal at 7.2 MHz is beat against the LO to produce audio output -- that is Direct Conversion. But in our example above we see that a 1khz output is produced as you tune two values of Local Oscillator input. A 7.199 MHz LO produces a 1 KHz signal as does one at 7.201 MHz. If this were a SSB signal the 7.201 MHz LO would produce the LSB signal and the 7.199 MHz the USB signal. So for those who have never operated a DCR --your first thought is there something wrong with this design or that you made a mistake. You didn't and it isn't a bad design. It is a simple receiver! Use the computer between your ears to sort through the signals.

 

Were you actually tuning the signals, the dial window of a Drake 2B would show the two signals ( as the two blue tick marks ) on the dial. (Thanks to G4WIF for the several graphics and photos!)

 

Now about LO's and I have listed the several which could be used with this project:

 

1
Arduino / Si5351
2
Arduino / Si570
3
Arduino / AD9850
4
Arduino / AD9833
5
Conventional LC Analog VFO
6
VXO

These are listed in Rank Order and a bit of an explanation. The number one choice is for the reason that the Cat-Around will morph into a more complex project (SSB Transceiver) where you will need both a LO and BFO. None of the other choices can produce independent multiple frequencies as a stand alone.

Choices 2 - 4 will work but are single frequency only. The Si-570 will be touted as having less phase noise; but also is more costly. Choices 3-4 are frequency limited in that the best the AD9850 can do is about 35 MHz and the AD9833 "poops" out at 12.5 MHz. Now if your interests are purely 160-30M, then the least expensive approach would be the AD9833. If you use choices 2-4 and move on to the more complex project then you will need to have a source for the BFO frequencies such as crystal oscillators.

Frequency stability in any rig is highly important -- the DCR adds another measure of complexity and that is the two sidebands -- thus you absolutely want it to stay on frequency. The conventional LC VFO is the least desirable (read poor choice) as it is very difficult to build a stable LC VFO beyond 10 MHz. The effort required to do so is not worth the investment in highly stable components nor the metal bashing needed to make mechanically stable plus add RF isolation. Get over the LC VFO and learn how to build a digital VFO.

Choice 6 is better than Choice 5 as you get variable frequency with the stability of a crystal oscillator. The obvious limitation the smaller tuning range. But that might be OK if say you were using this as part of a CW setup covering say 7.025 to 7.060 MHz. Now I have built a crystal switched heterodyne VXO where you use a higher frequency super VXO operating using three 12.228 MHz crystals and then using a NE602 (as a mixer) with a crystal oscillator injection frequency of 5.068 MHz. Boom, the output after passing through a Band Pass Filter is in the 7.160 MHz range works. Typically you might see a 50 kHz tuning range.

All is not for free! There are linearity issues as most likely the Super VXO would be varactor tuned.

But I have a 20M Shirt Pocket Transceiver which uses this approach. Here we use the 12.96 MHz VXO and mix that with 6 MHz crystals that results in a 19 MHz signal. The IF is 4.9152 MHz and thus the subtractive mix of 19-IF [4.9152 MHz] = 20 Meters USB. The schematic is shown below only to indicate how you could hook up the Super VXO and Fixed crystal oscillator. Ignore the ADE-1 as it was only for the 20M SSB project.

 

 

You will spend more time and more money getting the VXO to works (versus the Si5351) but you do avoid having to work with the Arduino and having to learn how to change programs.

 

In the case of Choices 5 or 6 you have the readout problem, whereas, the other choices may be used with a LCD or color TFT display. I have even "diddled" with the code so that what you read on the display would be 7.2 MHz and not 7.199 nor 7.201. The magic of software!

The very bottom line is stick with the Si5351 as that will give you the most flexibility in moving from the DCR to the Transceiver. The other options certainly will work for the DCR but will have limitations when moving beyond that to the transceiver

My choice is Option #1 and if you send me an email to n6qwham@gmail.com I will be happy to supply the code for the DCR .

 

 

 

 

A Blinking Sign should pop into your head! The source voltage of 9 VDC for the Arduino came from Mr. Arduino himself, Massimo Banzi. If you did read his book "Getting Started with Arduino" on page 18, he states when using the Arduino with a source other than a USB port USE 9 VDC! Many of you would be home brewer experts will say I use 5 VDC. Well it is like: Listen to Dr. Fauci and not some ex reality TV star about Covid19 Vaccines -- so Listen to Banzi!

So OK on to the less exciting part of the build and for these I will just include schematics, Shown below are the Band Pass Filters for 40/20M, The Audio Amp Stage and the 2N3906 RF Amplifier Stage

 

 

CONSTRUCTION NOTES AND HINTS

At the outset, how you build the MC1496 DCR is critical. The DCR cicruit is subject to noise and hum so great care must be exercised in using short direct connections and a very large common ground. The perforated board is not "plain vanilla" perf board; but is actually single sided copper vector board. The top surface is all copper. So a connection to ground with a through hole component is simply passed through a hole and then soldered to the board. For components not connected to ground I first ream away a bit of the copper around the pass through hole using a 1/8 inch drill bit. Non-grounded connections are made on the underside of the board which is non-conductive.

 

A bit of a trick here, look at the board photo and the MC1496 schematic as shown above. There are two 1K resistors connected to Pins 1 and 4. These two 1K resistors are phyically located to the right side of the socket near pins 7 and 8. Both leads pass under the socket (on the underside) with the upper 1K resistor connected to Pin 1 and the lower 1K resistor to Pin 4. Thus no crossovers and no shorts. The resistor closest to pin 8 is the 820 Ohms. Boom the other ends of the 1K's and the 820 Ohm are all connected to a third 1K to Ground. That is seen in the Photo as 90 Degrees to the other three resistors. This layout has minimum lead length and solid connections to ground. At pins 1 and 4 in addition to the 1K is a 100nF with the Pin 1 being the Signal Input from the RF Amp and the one on Pin4 is Grounded.

I have a piece of non copper coated board that I do trial runs to achieve the very best layout. Then I make a photo of the optimum layout and transfer that to the final board. THIS IS TRIBAL KNOWLEDGE AT WORK! Look for the parts I just mentioned!

 

I strongly suggest that you do an actual LT Spice Simulation of the Band Pass Filter Networks and also the PNP RF Amplifier Stage. You will note that as you simulate the BPF's the pass curves are quite sharp and there is a loss in the network. I personally like a network that keeps out the crud and if there is a loss, then compensate for that loss in other parts of the circuit. Once built, you can take your Nano VNA and actually test the Band Pass. If you have carefully wound the cores and used high quality caps (COG or NPO) then the VNA plot will match the simulation. [Note others beside myself did this exact exercise and so I am not the only one who has validated the design for the BPF using both LT Spice and the Nano VNA. So if yours doesn't match, you did something wrong -- the design is SOLID!]

Now as you will note when you simulate the BPF's they tend to favor the SSB portions of the band. But for those of you who are CW oriented what do you do? You could lift the 3 section design from the Bitx40 and use that for 40 Meters and if you think about it can scale those values to move it to 20 Meters or 75 Meters. Ouch that is hard for many or you can visit www.n6qw.com/GQRP.html and the designs are there. I am making it too easy for you. The 3 section designs tend to cover about 2 MHz versus about 300 kHz.

The PNP RF Amplifier is one half of the Plessey Bilateral Stage as shown in Figure 6.110 of EMRFD. So for those who only undertake circuits shown in EMRFD, this has a pedigree and passes the Smell Test. There, I won't have to defend why I used a PNP and why those circuit values. In the course of simulating this circuit, try changing the value of C1 and look at your plots. You will find that an interesting exercise.

 

Now the Fan Dancer of the 1920's (100 years ago)

The Fan Dancer of the 1920's were painted ladies who essentially removed their clothes in front of a largely male audience. As a part of the attire were large feathered fans, which were the one part not removed. Somehow these "entertainers" showed all; but you saw nothing because of the adept maneuvering of the fans.

 

 

 

The 2020 Version of the Fan Dancer ~ The Cat-Around

The MC1496 DCR is Now a Two Band MC1496 SSB Transceiver!