(Editor's note - I will eventually add an index for this series. Until then, please look in the Ham Radio Online Library for previous articles in this series... de Ed, KF7VY.)

It is possible to build your own DIODE RING MIXER, but it is an error-prone and time consuming task, so I have chosen to use a commercial version, the TUF-1 as shown below (bottom view). 

If you can’t find the TUF-1, the SBL-1 does the same thing, but has a very different pin-out arrangement, as shown in Figure 2. Pins 2,5,6,& 7 go to ground; pin 1 is antenna input; pins 3 & 4 are IF output, and pin 8 is VFO input. 

I like the TUF-1 because it is tiny, and will plug into a 4-pin SIP socket or one side of an 8-pin DIP socket. Some builders insist that diode ring mixers must be soldered directly into the circuit, but I always use a socket, and have had no problems. The mixer can, of course, be soldered directly into the circuit, if you prefer. 

I just KNOW you are wondering what’s inside the TUF-1). Figure 3 shows the "innards" of the TUF-1. 

The SBL-1 is the same circuit, in a larger package, with a different pin-out.

The transformers are sometimes drawn as shown in Figure 4. Yes, it’s the same circuit. I have labeled the windings so you can compare the two drawings. If you want to "roll your own", use 1N914 or 1N4148 diodes. The transformers are 8 TRIFILAR turns on FT23-43 cores. Phasing MUST be as shown. 

Diode mixers introduce about 8 decibels of loss into the signal path, so I have provided a buffer/amplifier to boost the signal, as shown in Figure 5.

T4 in Figure 5 is ten BIFILAR turns on an FT37-43 toroid core. Notice the phasing dots. The windings MUST be phased as shown. 

The values for R17, R18, and C22 are found experimentally. You can use the values I show, and your receiver will work fine, but it may not be optimum. If you demand optimum performance, and r-e-a-l-l-y like to tinker, use 10k pots for R17 and R18, and a 120 pF ceramic variable (JAMECO part #94465, or equivalent) for C22. I’ll discuss adjustments later.

About transformers: I always use a different color of wire for each winding, as shown in Figure 5, above, and in the illustration below. This helps to avoid confusion as to which-end-goes-where.

The layout I used for the receiver front end, including the IF amplifier, is shown in Figure 7.

Testing the 40 meter receiver front end is quite simple. As usual, double check for correct connections and good solder joints. Check resistance from the 2N5109 to ground: Emitter, 56 W ; Base, 470 W ; Collector, 1.47K. If resistance checks are OK, proceed to voltage checks.

IMPORTANT: The 2N5109 collector is internally connected to the case, so neither the case nor the heat sink should touch any other component.

Apply +12VDC and CHECK VOLTAGE from ground to the 2N5109: Emitter, 1.8 volts; Base, 2.6 volts; Collector, 8.2 volts. 

Once all the 40 meter receiver modules have checked out OK, it’s time to hook them together and test them as a unit.

Each builder will do things a little differently, but a general check list is shown below.

Turn off all power to the modules, then connect them together for testing ...

Connect an antenna and the VFO to the front end module.

Connect the output from the IF amplifier to the receiver "rear end" (product detector, and audio). 

Be sure there is a good ground connection between all modules.

Before applying power, set the BFO to about mid-range. Set the Main Tuning (VFO), IF Gain, and Volume controls to minimum (fully CCW). 

Apply power and adjust volume control so that you can hear a sound (either noise or signal). Advance the IF Gain control to just beyond where the volume starts to increase. 

Tune around the 40 meter band. If you don’t hear any signals, chances are you have a wiring error or bad solder joint because there are (almost) always signals on 40 meters. On the other hand, you may have picked one of those rare times when there are simply no signals to be heard. You can use a signal generator or a GDO for a signal source, or if you have a 40 meter transmitter, you can feed a signal from your transmitter into a dummy load and use that as a signal source. If all else fails, call a friend and have him (or her) put a signal on the air for you. 

Once you hear a signal (not noise), you are ready for adjustments in the receiver. Unless you have a workbench full of test equipment, this will have to be done by ear, which is fine because your ear is, after all, the final arbiter of performance. 

Find a CW or RTTY signal, NOT a steady tone. (A steady tone might be a "birdie" that is being generated within your circuitry.) When you have found a "real" signal, adjust the volume to a comfortable level. Next, tweak the IF transformer for loudest signal. NOTE: The tuning slug is brittle and easily broken. Be gentle! NOTE #2: This should not require more that 1 or 1 ½ turns in one direction or the other. If there is no change in signal level after one full turn in either direction, you probably have a wiring error or a bad solder joint. Find and correct the problem before continuing.

If you used a variable capacitor for C22, adjust it for loudest output. If you are using pots for R17 and R18, adjust them for best results, then go back and tweak C22. "Best results" means best selectivity. You may have to play around with R17, R18 and C22 for a while to get it right.

Your 40 meter receiver is just about done. There are a few more things to do, but functionally the receiver is (almost) complete, and you are ready to listen to the 40 meter CW band and experience the satisfaction that comes from using a piece of equipment you have built yourself.

Using my trusty military surplus signal generator as a VFO, I have tuned this receiver from 5 to 50 MHz. It makes a fairly good general coverage receiver. Some may be wondering how in the world you can get such wide coverage. The answer is quite simple; diode ring mixers are broad band devices. The TUF-1 is good from below the AM broadcast band to UHF. All you need is a VFO that will tune near where you want to listen. "Near" is the difference between the desired frequency and the IF frequency, which brings up a problem that alert builders, such as yourself, have noticed: broad band reception comes with a price. Namely, those pesky SSB signals right in the middle of the 40 meter CW band. We’ll get rid of ‘em, but first, where do you suppose those signals are coming from?. For those who may not have been watching closely, here is the explanation.

As you recall, your VFO tunes from 10.58 MHz to 10.74 MHz so you can hear 40 meter signals from 7.00 Mhz to 7.16 Mhz (give or take a few kHz, depending upon how you align your receiver circuits). The DIFFERENCE between the VFO and the desired frequency is, of course, the intermediate frequency, 3.58 MHz. 

Notice that the SUM of the VFO frequency and the intermediate frequency is 14.160 MHz to 14.320 MHz, which is right in the middle is the 20 meter phone band (The plot thickens!). Mixers produce BOTH a SUM and a DIFFERENCE frequency, so when a 20 meter signal comes into the mixer from the antenna, the VFO frequency is subtracted to give a 3.58 MHz intermediate frequency. That’s where those SSB signals you heard are coming from. Yes, this receiver works equally well on either 40 meters or 20 meters, all at no extra cost (clever, don’t you think). 

The problem is: we (you and I) are only interested in 40 meters, so we must tell the receiver which band we want. I’ll show you how to do that next time. ‘Til then…

‘Til then … 73, Dick, W6BKY