Reflecting Back In Time...
[N2CQR, Bill, reminded me that Direct Conversion Receivers were embedded in commercial product offerings such as the famous Heathkit HW-7 and HW-8. I also think one or two Ten Tec Rigs may also have employed that same DCR technology. I seem to recall The Century 21 as being one. But i might have that wrong. Elementary bare bones rigs like the PIXIE are in reality crystal controlled DCR's.
KK7B published an article on a phasing receiver that at the heart uses two DCR's and shows a technique for eliminating one of the sidebands.
There are of course many published articles from sources like the GQRP SPRAT and QQ that have documented such DCR projects. Below from Winter 2012 QQ is a FB article from the prolific and world renown author N6QW. Too bad it used an Analog LO which is a real detractor to this project.]
The prior photos that were posted in this run highlight and focus on the "artful" nature of those homebrew rigs. Skill was needed to blend the electronic hardware with common materials like wood. Regrettably you develop no construction and fabrication skills from the mere flashing the plastic at Amamzon.com
The wooden bases stand out and were likely painted and shined before final assembly of the electronic parts. The evidence is clear they simply looked "cool". I have tried to carry on that tradition with my painting of rigs with vibrant colors such as Juliano Blue or Julieyellow.
But what about receivers? The form of our receivers has taken an interesting journey form Coherent Detectors to the DDC. But one particular form popularized by W7ZOI is the Direct Conversion Receiver (DCR) whose roots reach back to the 1960's, which may in fact be older than some of the blog readers. Notwithstanding that the DCR was subject to hum pickup and you did get the same station at two places on the dial. It was indeed a staple of the QRP community. The received signals had "presence" as identified by another giant W1FB, Doug DeMaw.
As for me personally I have not built too many DCR's with the only ones being built were the one that was an integral part of the Lets Build Something Transceiver project where the DCR was morphed into a full fledged SSB transceiver. I also wrote an another article for QRP Quarterly that used a DGM as a DCR. BUT more recently I have built literally a dozen DCR's that are an integral part of the five SDR transceivers sitting in the N6QW Laboratories.
A pair of DCR detectors (two ADE-1's) form the mainboard of my homebrew SDR rigs. True I did address the hum issue with some modem coupling transformers and the software takes care of single signal reception. But the Bonus is that they transceive!
I like to take a moment to talk just a bit about how the DCR works. In simplistic terms I like to think of the DCR as a three terminal device. One terminal has the signal input off of the air. A second terminal has a local oscillator input and the third terminal is the Output port. Essentially the DCR converts the signal off the air into audio. The process is a lot more complicated than this; but for our discussion think In, In and Out. Because of the direct conversion process we find that we get the recovered audio at two places on the dial.
Say we have a 1khz tone transmitted on 7.2 MHz we can get a 1 kHz audio tone output at two LO Settings. Actually some of the resultant mixing frequencies are at 14 MHz; but are not passed on to the audio amp stage.
Most of the simple DCR's rely on the computer between our ears to sort out the right signals. Yet there are other ways to remove the unwanted signal; but often very complex signal processing involving a computer or the internal computer we all have in our heads to make that happen.
But why screw with DCR's and the answer is simple --yes Simple Circuitry. A four pin TUF-1 DBM has the necessary three ports plus ground on a device that fits on your thumbnail.
Because there is a conversion loss in a DBM like the TUF-1 you probably need some sort of a simple RF amp stage ahead of this (like maybe a use for our PNP transistor amp) and a Band Pass Filter since essentially any and all frequencies are being received. A strong audio amp to follow the DCR is a must. Forget configuring a 741 op-amp like an audio amp -- think like a 2N3904 driving an LM380. You can search this blog for LM-380 audio amp.
For those who would profess an interest in homebrewing something, a direct conversion receiver is a really excellent first step. W7ZOI built a project called The Weekender Special which was two separate units a DCR using a DBM and then a companion transmitter. Check that out.
Now I know there are other forms of the DCR including the use of a Dual Gate MOSFET such as the 40673 which can be simulated with two J310's. Earlier I mentioned my article in QQ about such a project.
Still other more sophisticated IC devices like the those from Motorola and Plessey that are used a Product Detectors are great candidates.
Gulp, you could even home brew a double balanced mixer with four diodes and two ferrite cores and that would work. There are three you tube videos from N6QW that show you how.
BUT for those new to homebrewing --Don't. It is hard to replicate a DBM built on a manufacturing line versus one you cobbled together in your hot garage shack. First there is the problem of truly matched diodes and secondly the winding of the toroids is critical. Do Not Believe all of the claims of amazing results with a homebrew DBM. I found that even this unit was lossy and versus being a 7 dBm device was more like 10 dBm which requires a higher LO drive level. It was deaf until we upped the juice. Later when you have a working DCR with a real DBM you can think homebrew.
This an example of a homebrew DBM that was used in the LBS Project and served both as a Product Detector and Balanced Modulator. This has some refinements like AF filtering on the output and a balance pot control. With a bit of additional components you can remotely unbalance the DBM such as you might want for CW generation. Yes Virginia you can find that in EMRFD from W6JFR (me).
Today's post is to get you thinking about doing something other than ordering parts from Amazon.com and using tools other than your plastic card.
73's
Pete N6QW
Very Early Rigs looked maybe like this.
It is said that Denny's will give you a free meal if it is your Birthday. But if it is your Birthday and you are in Denny's for that free meal then possibly your Life Sucks! [This was on a Bumper Sticker and it caught my eye!]
At times when we look at the photos above we think of simple times and yet fun times. The cost was not large; but the results were stunning. Yes I did build one or two of these elegant rigs back in the day. Some of us actually yearn for a return to those days when all hams acted in a courteous manner.
Just listen to the foul language on 75 and 40M or as I heard today from some hams in Arizona staking a claim that Senator Harris in not a citizen and therefore could not run for VP. It was Gospel as they heard it from the emperor himself. Wow, there really are ignorant people out there and many are hams.
Simpler times and simple technology are a lost dream. No Menus and no fancy $300 microphones. Just simple rigs that made contacts. CONAR Receivers and Transmitters were kits offered by NRI that really was a mail order technical school that sold the kits as a part of the course offerings.
The pair were triband and pretty rudimentary. For a mere $400 you can now buy both of them on eBay. For a lot less money you could actually build them. Oh forgot, a significant segment of our ham brethren do not build things -- I guess that is why the price is out of line.
Take a moment to think if you were to build a rig what would it be?
I just heard Texas has over 10K deaths from Covid19. Our hearts and prayers go out to all of those grieving families. But two people are responsible for this: Donald J. Trump and Greg Abbott! [Let us also not forget Mike Pence.]
Their lack of leadership is the core issue. Implementation of a national mask policy would have avoided many of these needless passing's. November 3rd is your chance to change the landscape!
I just heard Texas has over 10K deaths from Covid19. Our hearts and prayers go out to all of those grieving families. But two people are responsible for this: Donald J. Trump and Greg Abbott! [Let us also not forget Mike Pence.]
Their lack of leadership is the core issue. Implementation of a national mask policy would have avoided many of these needless passing's. November 3rd is your chance to change the landscape!
73's
Pete N6QW