Hello, I am hoping I can get some accurate information here, regarding several questions I have about the Chris Cuff C-Quam AM Stereo Transmitter.
My first big question is regarding the transmitter's output impedance to the antenna.
I have schematic diagram images of the transmitter's output stage, so that should help with anyone answering this post, knowing how it is designed. By advised, permission was granted to me by the author Chris Cuff to share this and any schematic diagrams on forum boards.
First, let's look at an image of the schematic diagram of the RF output stage:
Now, because the Chris Cuff transmitter is built was originally intended to be a novelty item, the output stage is very, very limited!
My intentions are to use a MW RF linear amplifier, that has a 50 Ohm impedance input.
The last RF amplifier transistor that was used on the final stage of the Chris Cuff was a 2N3035 transistor. looking at the schematic diagram image above, that transistor, is located where the LAMP is shown in the diagram at bottom left corner. The schematic diagram lists 3 other transistors that have been known to be used as the output transistor.
My first quest is to resolve an issue with a mismatch to the 50 Ohm impedance of the linear amplifier's input. This mismatch is causing loss of signal to the amplifier's input and issues with amplifier running hot.
The specks for the amplifier are as follows:
100 KHZ - 30MHZ RF Linear Amplifier, indicators are as follows:
The working frequency of 100 KHZ - 40 MHZ The input power 0 DBM 37 DBM output power, that is, 5 watts Harmonic suppression (typical) - 20 db Working voltage 12 -- 15 v The working current 0.8 A Input and output impedance 50 Ohms
Now, I have tried to figure out when using the 2N3035 type NPN transistor as the final, what is required to get this thing to produce a 50 Ohm impedance at the output, to satisfy the mismatch and have efficient input to the amplifier.
I would also like to know what is the best NPN transistor to use at the output stage of the Chris Cuff transistor.
Besides the 2N3035, these other transistors are also listed in the schematic diagram, as ones that have been used in this transmitter:
1034-01
2SC1237
Sk3103A
Which transistors produce the highest gain?
By the way, for now, the working AM frequency is 1250KHz, I might change the frequency at a later date, but this also requires the changing of several other components, if the desired frequency is above 1605KHz.
Also, Krocks sent me some information via email, he is known here as RFBurns. He sent me these directions, to which are not clear to me, because he mentions MICA caps of .1 it is not clear to me what is meant by that value.
Below is the full instructions that were sent to me, besides the .1 mica caps to which, I can NOT locate on line anywhere (unless I am misunderstanding something here, I am also lost as to how I am going to know what the device impedance is where he speaks about making a BALUN, for example, if a 2N3035 transistor is used, how do I know what its output impedance is? Data sheets don't seem to reveal the output impedance of the transistor.
Now I have a Zenith 1034-01 transistor, I looked at a spec sheet for this transistor which has a cross reference of an NTE171 transistor, the specs on this transistor looks like it has much higher gain than the 2N3035 transistor, would that NTE171 or in my case the Zenith 1034-01 transistor be the best choice to use as the final transistor of the Chris Cuff transmitter?.
I would take based on that information above that the NTE171 has an output impedance of 10 Ohms? If this is correct please let me know.
Anyways, in bold below are the total instructions that Krocks (RFBurns) gave me. I also tried looking at 6 different AM radio receiver tuning cans, with the RED slugs as mentioned below, each tuning canister has 5 pins, Krocks did not tell me which of the 5 pins to use and each canister has a slightly different electrical configuration in the underside of the each can, so I do not know if that makes a difference.
Here are Krocks Instructions:
Hi Bruce. I'll be happy to answer questions on that unit. It is a good little transmitter/exciter on it's own if it is to be feeding a simple wire antenna or a loaded coil antenna/atu combo. The carrier it produces is well above the noises it generates from the digital circuits. However since that unit's output is not a tuned circuit, whereas the tuned circuit is the antenna system, driving amplfiers becomes an issue because of two things.
1. The fact the output is un-tuned.
2. The un-tuned output when connected to a set impedance input such as 50 ohms, loads down the signal level at that output to almost the same level of the noises from the digital circuits...thus you end up with a very dirty signal and not a pure carrier signal and all that will pass thorugh to the amplifier, which is why the difficulty of impedance matching and lack of carrier drive.
Your first step is to eliminate the hash noise with the .1 mica caps from each B+ point at each IC to ground. Then install 100uH to 300uH chokes at the voltage regulator in's and out's. Third thing to do is use local oscillator coils found in AM radios...in this case the red slug coil. Using 2 of these coils and 3 fixed capacitors (or variables for finer tuning), your forming both a impedance matching network and a low pass filter...a tuned output. Caps to use here are 1000uf/3000uf/1000uf. The / are your tuning coils. See attached drawing.
The other option for the output is to use a balun transformer. This approach would turn the output of the unit to a "broadband" output. But you want to make sure that all of the filtering described above is done to eliminate all the hash, otherwise it will still be present at the balun output.
The balun would have to be wound with the output device impedance in mind for the balun input. So if the output device has a 10k impedance, you will need a high impedance input on the balun...ie a lot of windings. Using a TO-32 core wind #22 solid insulated coper wire 40 turns for the input. Then wind same wire type in between the first set of windings 10 times for the output.
You can combine both the tunable filter and the balun. Put the balun at the output of the unit, then the filter at the output of the balun. Tune filter for maximum drive into the amp.
What would also improve things is the last little driver stage before the 1 watt transistor had some tuning as well. Unfortunately there is no room on the board to install a tuning coil tuned circuit.
Anyway these initial steps can drastically improve that unit for amplifier driving.
Here is Krock's image for the two AM radio receiver tuning canisters and 3 electrolytic capacitors No mention is made as to which of the 5 pins on those cans should be in the circuit, so that makes it confusing to try that circuit below.
Okay I'll leave this post as it is, before I add to much to anyone's plate who is willing to answer.
PLEASE keep in mind, I am NOT advanced in this type of electronics regarding building RF Power Amplifier stages, this is why I bought a pre-built 5 watt MW linear amplifier, that requires a 50 Ohm impedance, both in and out and I want to feed the transmitter into the amplifier to amplify the RF power output for greater range.
The .1 capacitors are basic 0.1uF ceramic (no idea why he suggests mica as it's unnecessary and adds more cost) used as decoupling capacitors. It will reduce any IC switching noises and reduce RF feedback along circuit paths. If the transmitter has neither of these issues then it can be overlooked for now. Power going into the transmitter can either be decoupled from the power supply circuitry as he suggests using RFC coils, or you can use a toroid good for low frequencies and just coil your power wire(s) through that enough times to remove any RF getting back into the power supply or regulators.
The impedance matching network is basically what is required to transform the unknown impedance of the transistor output to a 50ohm standard impedance to drive normal antennas or linear amplifiers. It will also help smooth out any harmonics generated as it will act as a filter. Yet another quick and dirty way to do this is to feed the output to a 50ohm resistor to ground like a dummy load by using a coupling capacitor between (so not to electrically ground the transistors collector) and then on the same leg of the resistor run it to your linear amplifier. This will only work with RF voltage driven linear amplifiers though, like MOSFET linears where the gate is mostly driven by voltage and not current. If it sounds wasteful on dumping RF into what amounts as a dummy load, that's because it is, but with MOSFET linears they aren't current driven amplifiers anyways unlike bipolar types, so that could be an option to try depending on how your amp is designed.
As far as the transistor used in the circuit you posted above I would be careful about swapping it out for other types unless you have a way to monitor the modulation on an oscope since it's only biased by that one 56k resistor for linear operation itself. When using other transistors its bias may need the value of that resistor changed so to be fully linear swinging equally positive and negative with respect to its modulated RF input.
With all of these linear amplification stages your efficiency is going to be quite low so don't expect much RF power output but plan on lots of heat being generated. This is why low-level amplitude modulation is more suited to low power transmitters. If you want to run much more than tens of watts you almost have to use high-level modulation at the last RF amplifying stage and run everything class C up to that point. Just food for thought.
I am trying to get a picture up here of the linear amplifier internals, but image size limits keep getting reached, no matter how I save or convert the image to a smaller size.
Okay this is what the linear amplifier looks like inside, note, yes, the company scratched the ID off the main transistor, but at least you can see what type it is.
Wow WTF at what they did to that transistor! It's like they took a dremel tool to it and just gnawed away at it. Nothing like taking half the damn transistor mold off almost exposing the internal guts. That amp from looking at the picture looks like it may already have a LC network on its input but it's hard to tell, and especially difficult given the part numbers are gone, but I suspect it's a common circuit. The output transistor looks like it's in a gate/source/drain or possibly base/emitter/collector configuration from left to right by looking at the pins. The blue precision resistor more than likely controls its bias for class A amplification. I'd imagine it runs quite hot depending on the part used. Top 3 leg part has got to be a voltage regulator for biasing voltage and left 3 leg part is the input transistor. It does look like it's LC coupled from input to the input transistor so there may be a few ways to drive it but you won't know until you experiment.
By the way are you sure that RF amplifier is suitable for medium wave work? Being that it says on the PCB that it's an HF amp but I do know some of them are good down to the top end of the AM MW band. The RF choke on that board looks to only be a 100uH which makes me wonder if it's suitable for such low frequencies. I better not reverse engineer that board from your pic too much though or the original designer may see this post and decide to encase the whole damn thing in an unbreakable plastic mold
I am waiting for a second round of transistors to be delivered from the amplifier seller.
I blew the first transistor in the linear amplifier, which is in the amplifiers input circuit stage.
See my image below showing the linear amplifier internal view, (Linear amplifier input stage is on the left hand side of this image, output is on the right side of photo) the first transistor after the two SMD caps was shorted out and defective.
Seller sold me a second transistor, installed it, but made a major error and blew that one also.
The issue is, I do not know what type of transistor it is (there is no marking on the transistor). Under testing it does not test like a conventional bipolar transistor, using an ohm meter and transistor checker, it does not test like a bipolar transistor.
I have spoken to the seller of the amplifier, asking for a little bit more information about the transistor, but was told the manufacturer chooses not to disclose the parts identification numbers, as can be seen by the removal of one such transistors identifier numbers in my photo above.
I have three of those transistors on route from China, so I have 2 extras in case something goes wrong again.
I realize what I did wrong with the 2nd replacement transistor, is I had by-passed the coupling capacitor and fed the 2N3053 transistor output directly to the amplifiers input.
This sent (+)9 volts DC from the Chris Cuff Transmitter directly into the amplifier's input stage, shorting out the first component in the input stage.
I was trying to figure out what Pico farads range, was best suited as the output capacitance, it appears that a 150 pf cap worked well, sadly, I by-passed that cap temporarily and sent DC into the amplifier.
I do not have a meter that works well in the AM broadcast band, but I do have a Radio Shack Micronta CB SWR/Watt meter handy.
When I had the 150pf cap at the output stage of the Chris Cuff and the amplifier input and feeding into the meter with a 50 Ohm dummy load at the output of the meter, it showed a 5 watt reading on the meter in the 0 to 20 watt range and an SWR of 0:1 Perhaps the meter is covering with some accuracy the AM broadcast band, because, the linear amplifier output power is supposed to be 5 watts anyways.
I am going to post am image below of the total output stage of the Chris Cuff AM C-Quam Stereo transmitter, I want you to study it and observe how it is designed and comment on any issues you see with the circuitry.
Although the diagram shows a list of RF POWER AMP STAGE transistors, the one I have in that location is an NPN 2N3053 bipolar transistor, just so you know which one I am working with in this transmitter.
Mind you, just before the FINAL transistor, the design has two capacitors in series.
A 0.047 capacitor at point G feeds into a 150pf capacitor, if the output stage labeled RF POWER AMP STAGE is used.
I am not sure if the 0.047 capacitor is placed in the circuit if the antenna is placed at area G and the RF POWER AMP STAGE is not used and should be removed if the RF POWER AMP STAGE is used, thus either the 0.047 or 150pf should be removed from circuit if the RF POWER AM STAGE is used.
I took the liberty to remove the 150pf capacitor, so the 0.047 capacitor feeds the base of the final transistor and the 56K resistor junction. Then placed the 150pf capacitor on the collector of the 2N3053 transistor to antenna output, to linear amplifier input.
Not sure if a 150pf capacitor is a good choice here, I have a large assortment of disc capacitors, of different types, but I am open to suggestions as to what capacitance I should use here.
Here is the current diagram of the Chris Cuff AM stereo transmitter pre-driver and output stages, please look it over carefully and tell me if you see any issues.
BY THE WAY AT THE +5 VOLTS AT REFERENCE POINT F "FROM ENVELOPE MODULATOR" IN THE SCHEMATIC DIAGRAM WAS MOVED OVER TO THE +9 VOLTS CIRCUIT BY MOVING TWO RESITORS FROM THE +5 VOLTS CIRCUIT TRACE, OVER TO A NEAR BY +9 VOLTS TRACE, THOSE TWO RESISTORS ARE THE 15K AND 560 Ohm RESISTORS THAT FEED THE FIRST 2N3904 TRANSISTOR IN THE SCHEMATIC DIAGRAM INCLUDED BELOW.
I did not want to sound like I was yelling in that last paragraph, but I wanted the question to stick out more! I would like to know if changing the +5 voltage at circuit trace F, to +9 volts at this stage of the circuitry is acceptable and was a good move or not, or is it better to keep it at +5 volts? (Please see circuit trace F 5 vdc in the diagram below)
I'm a bit lost as to what you're trying to achieve?
The circuit looks terrible BTW and not sure why anyone would think of using it.
Are you wanting an A.M. transmitter for MW?
How much (carrier)power do you need?
What PSU do you use 12V, 50V?
Let me know.
I can help!
Stretchy.
Strechy, what I am trying to achieve is broadcasting in the M.W. band in C-Quam AM Stereo.
I already have the Chris Cuff transmitter fully assembled.
The issue with the transmitter is it very, very low RF power output.
I am trying to boost the signal using a M.W. 5 watt linear amplifier.
The transmitter does not have an actual impedance matching circuit at its output stage, because it was designed for a 10 foot wire antenna and a copper wire coil that is 100 raps around a 1/2 inch plastic tube cylinder and variable capacitor for antenna tuning.
The linear amplifier requires a 50 Ohm impedance at both the input and output terminals. A mismatch at either end causes amplifier issues, including over heating the transistor.
I realize everybody wants to sell something to make other people's lives easier, such as you wanting to sell me another amplifier. Either way, the schematic diagram I posted above is for the final RF power output stage of the Chris Cuff transmitter. It puts out nano-watts and was intended as a novelty item, not an actual FCC part 15, 100 milliwatt transmitter.
I am trying to boost that signal and feed it to an outdoor antenna, with 10 maybe 20 feet of 3/4 inch copper pipe and base mounted tuning coil.
First I have to get that impedance up or down to 50 Ohms at the output stage of the Chris Cuff transmitter before feeding it into the linear amplifiers input stage.
I do not possess any inductance meters at the present time to do various LC or inductance trials and errors, so I have to rely on known circuits that have been known to work. Otherwise, my amplifier will simply over-heat and or fail do to improper loading.
That is the help I am asking for.
I am waiting for 3 replacement transistors I ordered from the amplifier seller to arrive, which should be here by tomorrow. I burned the last two input transistors by doing trail and error, accidently sent DC current into the amplifier, by not having a coupling capacitor in between the output transistor and the amplifier input to block DC 8 volts from getting into the RF input section of the amplifier. The way the Chris Cuff final output transistor is power, see schematic diagram in my last post, 8 volts is present at the emitter of the final transistor, a tuning capacitor usually blocks that when the tuning capacitor is present inside the kit enclosure, but when attempting to add an RF linear amplifier, this makes hook up a bit different since the tuning would now be done between the amplifier and actual outdoor antenna, which would be feed via a 50 Ohm coaxial cable, possibly 100 feet in length to reach antenna location.
I may just try a 150pf cap at the output of the 2N3053 transistor (at the Emitter) and feed that into the 50 Ohm patch cord to amplifier input and hope for best.
I have been told by Chris Cuff that the more DC current you push into his transmitter, the further it will put out, however I have to question how that is possible if the regulators which are 5 volts and 8 volts would pass 24 volts or even 30 volts as he claims. I would think, eventually something has to give at those higher voltages and BOOM!!
Hi Bruce, firstly I don’t sell amplifiers but do sell transmitters that do work and work well. Looking at the design as with many others I’ve seen I’d pass trying to make it more powerful as it’s frankly a waste of time. Design a TX for the power you need and with ALL decent designs RF wise everything needs a 50 ohm impedance. To measure impedance you need a Network Analyser BTW something No RF lab should be without along with a ‘Scope, Spectrum analyser and a proper power meter and a set of attenuators. The list goes on but these are essential instruments to enable to ‘SEE’ what’s going on with a circuit. A ‘scope is an absolute minimum for most projects and just hope you have one? I piddled round for years blowing things up then gave it up because I got fed up with it all but now work for myself on radio related production and built TX’s in my spare time. I’ve found your post on another forum with the rest of the circuitry which looks fine, just forget the final RF amp as it’s silly, build a proper one, simple as. I’m not going to recommend anything however as there’s a wealth of mush out there that frankly I can’t be bothered with anymore as most of it is rubbish, hence why I decided to build my own. I’d stick to go ol’ AM if I were you!
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