Dusting off the crystal scanner

Do you have a crystal scanner that you just can't find a crystal for? Perhaps you have one that has your area's old analog public safety frequencies. Not sure what to do with it except throw it in the trash? Well today, I might just have the solution to keep it and the radio itself out of the landfill.

The Idea

If you understand the principle of heterodyne radios, then this should come as no issue for you. If you're not sure how it works, an RF signal is passed through a mixer which is mixed with a Local Oscillator (LO) frequency. Combining these two frequencies gives the Intermediate Frequency (IF) as the result, which is "the sum, the difference, and the two originals." Heterodyning is how we are going to make that old crystal scanner of yours work again!

It's a simple idea - an RF signal is mixed with an LO which is provided by a frequency generator such as the Si5351 or an analog VFO. The result will be fed into the crystal scanner's antenna input and will add up or subtract to one of the crystal frequencies your scanner has!

Take my Wards GEN856. Channel 2 has a frequency of 156.210MHz, so we'll use that as our intermediate frequency, just as an example. 156.210 used to be the local sheriff's department channel, but they have since switched over to P25. 

Now, let's say I wanted to listen to the APRS network on 144.390. I could go through and realign my scanner to the 2m and 70cm bands, or I could add another heterodyne stage to the scanner's antenna input. Or perhaps I wanted to listen to the local weather radio on 162.425MHz. Yes, I could have a custom crystal made, but wouldn't it be cheaper and a good learning experience if I just came up with a better solution?

Quik Mafs

The math is quite simple. The frequency we want to listen to (144.390MHz) will need to have a frequency added to it to get our IF frequency (156.210MHz). So we take 156.210 - 144.390 to get an LO frequency of 11.82MHz. In theory, this should work.

HOWEVER, our mixer will also produce the opposite frequency as well. Here, we are looking for the summation frequency (144.390 + 11.82), but there is a chance that we'll get the opposite image frequency (144.390 - 11.82). Again, remember that mixers produce "the sum, the difference, and the two originals." However, for as complex as our scanner is, it should lock on the frequency that is closest to 156.210MHz. 

Similarly, we can take our weather frequency (162.425MHz) and add it to our LO frequency. So we'd take 162.425 - 156.210 to get 6.215MHz.

But wait...if we add 162.425 and 6.215MHz, we'd end up with 168.640MHz! That's not right!

Ah, but remember..."the sum, the difference, and the two originals."

We can SUBTRACT 162.425 from 6.215 and then we'd end up with our desired 156.210MHz frequency. Problem solved! It's just how mixing works :)

Mixer Mix Up

Now there are several mixer circuits you can use. Most are readily available. For my project I will be sticking with the AD831 mixer, as it is common and easy on the pocketbook.

The only drawback with the AD831 is that the IF output is good up to only 200MHz. So if you are planning on using it for listening to UHF, consider another option. There are plenty on sites such as Amazon and eBay.

First, we need to feed our antenna input into the RFin pin. If you would like to amplify weaker stations, a receiver preamp can most certainly be used. Next, we'll need to feed our Si5351 output into the LOin pin. Finally, feed the IFout pin into your scanner's antenna socket. This can be done using a short run of coax cable, a ring terminal, or however you see fit for injecting the converted signal.

Receiving ham and VHF signals aren't the only things you have to listen to either. This method and principle should work with all frequency bands, with FM modulation of course, provided that your LO can generate a wide bandwidth of frequencies. 

More Info on the 856

About 4 years ago, I posted a question to the Antique Radio forum board asking if anyone had heard of this radio and when it was made. 4 years passed and I finally got an answer. This radio was first made in 1978, the same year my dad was born!

Of course, almost 50 years later, it still works. It has a 10.7MHz IF along with a 455KHz IF. And I was correct - this radio does triple the crystal frequencies to achieve reception on the crystals' overtones. The four scanner lights are operated by two NAND gates.

If you have a similar crystal scanner, give this frequency reception method a go. Most up and downconverters work the same way. This will most definitely give your old 1970s and 1980s crystal scanners new life!

In conclusion, the radio will work like this:

(162.425 - 6.215) [Mixer up/down converter fed into scanner antenna input] => ((156.210-10.7)/3) [Original 1st Stage IF and mixing circuitry] => (455KHz) [2nd Stage IF] => Demodulated Receive Audio

And this doesn't just apply to crystal scanners - if you want to listen to the low HF bands on something such as an RTL-SDR dongle, the same principle applies. This type of up and down conversion should work on any and all receivers. So go ahead, give it a try and see if you can bring your crystal-controlled scanners back to life!