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Need help with my RF design

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whiz115

Member
Hi

I did a small project so i can improve my knowledge and skills on RF and PCB designing without the use of any specialized software.. but seems i'm getting no where since i can't make it work as it is supposed to be.

initially i had oscillation problems with the regulator... the voltage was wrong..i could measure ~1,65V and i couldn't easily catch the transmitting frequency on my radio and if i did probably it was only harmonics and i could hear a high pitch sound which it was shifting up and down if i was touching anything on the transmitter.

The regulators i tried were 2 different LDO that i had available

Reg102-A
Reg101-33

https://www.electro-tech-online.com/custompdfs/2009/07/reg101-a.pdf

The problem persisted and i was thinking to try an LM2940-5 until i placed a ferrite bead in series with the (+) input power rail of the Reg101-33 and the problem i think is solved, the ferrite bead is a "Murata BLM31AJ601SN1L" which presents 600ohm impedance at 100 MHz and i had few available from past projects.

BLM31AJ601SN1L - Murata - datasheet

now i can measure 3.28V at the regulator output but still i can't manage to catch the transmitting frequency on my radio, for the LC tank i'm using an 100nH axial inductor so i can be sure that the inductance is somewhere near the needed value.

I also tried some simulations on Ltspice and multisim..but i didn't managed to make any succesful simulations..
 

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marcbarker

New Member
If that's an LDO reg you're using, it may be one of the LDOs that require a resistor in series with the regulator output capacitor to 'spoil' the regulation a little bit. It does sound counter-intuitive I know, to put a resistor in series with a filter capacitor, but if the capacitor is too 'good', the regulator 'hoots' (oscillates) and the output voltage average is rather low too. Best check the datasheet for the LDO, it'll say in there whether there's special requirements for the output capacitor.

Another thing I notice, is isn't there supposed to be a second 47K between base and ground? (not just the one 47K)

The other thing that's frustrating with transmitter circuits when you're inexperienced, is that you don't know if it's making much RF power or not. Or what frequency it's on even. Most hobby oscilloscopes won't even register 100 MHz.

A great piece and a very useful piece of RF test equipment is an RF sniffer. Get a moving coil microamp meter, and wire two small schottky diodes in series, connected between the meter's + and - pins. From the centre tap, you can either touch it on the circuit and see if there's RF present, or you can attach a piece of wire and make a field strength meter so that you can tune for stronger signal.

When you're using a radio to monitor, ensure that the antenna is down, or unplugged, because you'll have a better chance of finding the 'real' signal, rather than blasting through an extended antenna and picking up an 'image'.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi

I also tried some simulations on Ltspice and multisim..but i didn't managed to make any succesful simulations..


You must have a ground on the LTspice circuit.!

EDIT:
Try this sim as a starter.:)
 

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whiz115

Member
guys thanks for your interest :)

Best check the datasheet for the LDO, it'll say in there whether there's special requirements for the output capacitor. .

why don't you? :) it is suppose that i am the newbie here who can't even read a datasheet... that's why i gave you all this material and the links to the datasheets.

Another thing I notice, is isn't there supposed to be a second 47K between base and ground? (not just the one 47K).

can you explain further? first of all my schematic it is based on Audioguru's design...most of the circuit it is taken from his design.

https://www.electro-tech-online.com/threads/fm-transmitter-mod4.24887/

The other thing that's frustrating with transmitter circuits when you're inexperienced, is that you don't know if it's making much RF power or not. Or what frequency it's on even. Most hobby oscilloscopes won't even register 100 MHz.

A great piece and a very useful piece of RF test equipment is an RF sniffer. Get a moving coil microamp meter, and wire two small schottky diodes in series, connected between the meter's + and - pins. From the centre tap, you can either touch it on the circuit and see if there's RF present, or you can attach a piece of wire and make a field strength meter

you mean something like that?

https://www.electro-tech-online.com/threads/help-with-rf-diode.34306/

You must have a ground on the LTspice circuit.!

EDIT:
Try this sim as a starter.:)

yea! i know about the ground.. but it was a messy simulation anyway... i couldn't find the right components and the appropriate values...Ltspace can't find the LM317 so your simulation also can't work correct. can you help on that?

i also tried on multisim but i had similar results...
 

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marcbarker

New Member
why don't you? :) <..> read a datasheet...
If you post a link to d/s, someone, or me can oblige.

can you explain further? first of all my schematic it is based on Audioguru's design...most of the circuit it is taken from his design.
This transmitter design is very very old. I built one very very similar 30 years ago, and back then it was the same 2n2222 as well. Over the years I'd expect it to evolve a bit, but I wasn't expecting to see the base of the 2n2222 with only one resistor. This circuit used to have a 10K up and a 10K down from the base in the 1970's. Perhaps it was done that way back then because transistors used to have varying hFE and they are more stable nowdays.

you mean something like that?
https://www.electro-tech-online.com/threads/help-with-rf-diode.34306/

Yes that kind of thing, but not 'done-to-death' with discussion, instead just try it for yourself and you decide what works for you. I used lead-ended germanium diodes (OA81 I think) and they worked fine. If I wanted more sensitivity I would clip a piece of wire to the meter - as well as the wire antenna. I never ever had low sensitivity problems. I used one of these to develop a UHF TV transmitter in the early 80's.
 

marcbarker

New Member

REG101 series says it's a: 'new generation LDO that has this magical DMOS pass transistor, to give freedom from output capacitor constraints'. It also says it doesn't need an output capacitor. Yeah yeah ok, but that's down-hill with a wind behind it, that'll under be good test conditions.

But reading between the lines, if the output current is only a few mA (note there's 0.65 mA idle current), it probably goes unstable if the output capacitor is more than 10 uF, and if it's 'too good' a capacitor. Fix this by replacing any fancy tantalum with some crappy electrolytic instead, or if you can't find one put 0.1 Ohm in series.

I just looked at the ferrite data. Ferrite is magical stuff, solves lots of problems. It's under-rated.

Just wondering, why was AJ6 ferrite chosen? Any special reason?
 

whiz115

Member
But reading between the lines, if the output current is only a few mA (note there's 0.65 mA idle current), it probably goes unstable if the output capacitor is more than 10 uF, and if it's 'too good' a capacitor. Fix this by replacing any fancy tantalum with some crappy electrolytic instead, or if you can't find one put 0.1 Ohm in series.

if you noticed my schematic there is one 1uF capacitor at the regulator output... and yes it's a crappy cap.
there is also one more capacitor 100nF at the input and one more 10nF connected between the NR pin of the regulator and ground.

but...are you aware that small capacitors by their nature they register high esr values?!

check this link...

https://clientes.netvisao.pt/greenpal/meter1.jpg




I just looked at the ferrite data. Ferrite is magical stuff, solves lots of problems. It's under-rated.

Just wondering, why was AJ6 ferrite chosen? Any special reason?

what do you mean? the specific ferrites are what's left from an old project of mine... it was suppose to reduce EMI/RFI on the power rails of a DAC.

so...now what am i suppose to do so i can make the FM transmitter to work?

P.S the 47K biasing resistor at the oscillator transistor must be lower value or higher value when the regulated power supply is 3.3V?

the feedback capacitor at the oscillator transistor substracts or adds to the capacitance of the LC tank? how can i accurately know the resonant frequency for my LC tank?
 
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marcbarker

New Member
the specific ferrites are what's left from an old project of mine...

47K biasing resistor at the oscillator transistor must be lower value or higher value when the regulated power supply is 3.3V?

the feedback capacitor at the oscillator transistor substracts or adds to the capacitance of the LC tank?

how can i accurately know the resonant frequency for my LC tank?

Left over ferrites, OK. Well it seems to work.

Don't know. Depends on what the gain of that particular transistor is. When there's only a single base resistor (as opposed to 2 off), the circuit is more sensitive to variations to transistor gain variations. Also, the circuit is more sensitive to supply voltage variations. But there is the advantage it is a simpler circuit. I notice the emitter resistor is still 220 ohm after 35 years, some things never change. Measure and report the DC voltage on the emitter resistor, then I'll tell you. If there was 2 base resistors the circuit would behave better.

The 5pF feedback I beleive adds to the capacitance. I'm not sure, it's been a long time since I did this circuit.

Easiest way I found of measuring the res frequency is 'dip-it'. By using a hand-held Grid-Dip Meter that sweeps through frequency until it hits the tank frequency. When the osc is powered it'll be a slightly higher frequency, because the transistor is a _Varicap_, with a voltage on it. A calibrated wavemeter is a good idea to have, they do not 'lie' like a radio can.
 
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whiz115

Member
Measure and report the DC voltage on the emitter resistor, then I'll tell you. If there was 2 base resistors the circuit would behave better.

right now i got a 15K biasing resistor the regulated voltage for some reason dropped to 2.80V and after few minutes it went again 3.28V...the voltage to the emitter resistor is ~920mV


Easiest way I found of measuring the res frequency is 'dip-it'. By using a hand-held Grid-Dip Meter that sweeps through frequency until it hits the tank frequency. When the osc is powered it'll be a slightly higher frequency, because the transistor is a _Varicap_, with a voltage on it. A calibrated wavemeter is a good idea to have, they do not 'lie' like a radio can.


you mean a frequency meter? my english aren't that good... if you mean that...i had one Lutron LC-1200 available few days ago...
 
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marcbarker

New Member
right now i got a 15K biasing resistor the regulated voltage for some reason dropped to 2.80V and after few minutes it went again 3.28V...the voltage to the emitter resistor is ~920mV

you mean a frequency meter? my english aren't that good... if you mean that...i had one Lutron LC-1200 available few days ago...


Maybe RF is reaching the reg and it's over-reacting to it? Just maybe. If so, a ferrite might save the day. Also, is the supply rails of the oscillator decoupled together, decoupled separately from the reg IC?

920 mV is reasonable. You can get more or less RF by changing this voltage by altering the 15K. You usually get as much RF as you can.

No not a freq meter, an 'absorption wavemeter', it's basically a sweep tunable LC, with a meter, they typically have plug-in coils to cover a large range. A grid dip meter is kind of an AW with an oscillator added, with some means of measuring variations in power. A freq meter is still a good tool to have.

The 4.7 pF cap can be an adjustable capacitor.
 
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whiz115

Member
Maybe RF is reaching the reg and it's over-reacting to it? Just maybe. If so, a ferrite might save the day. Also, is the supply rails of the oscillator decoupled together, decoupled separately from the reg IC?

decoupling is as you see it on the schematic... should i add one more ferrite? where should i add it?

920 mV is reasonable. You can get more or less RF by changing this voltage by altering the 15K. You usually get as much RF as you can.

i forgot to tell you that the actual resistor on my circuit is 270ohm i don't have 220 so i'm using the closest value i have.

If Audioguru was around...he would say "change regulator!!" but i don't see the point of doing that... let's say i'll get an LM2940-5 then the oscillation will dissappear?!

I'm a Texas instruments child... i'm expecting one day they offer me a job! i'm kidding! :D
 

marcbarker

New Member
Before you condemn the LDO, stop the oscillator (press a 0.1u capacitor between E & C of 2n2222) and see if the LDO voltage becomes normal, that way you'll know that it's RF doing it.

I don't see much decoupling across supply rails, near the osc. The RC tank has some RF currents and there's no path for them to the 270 ohm, except through the LDO maybe.

Try put ferrite just upstream of decoupled osc. supply rail.
 
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whiz115

Member
Try put ferrite just upstream of decoupled osc. supply rail.

where is the upstream? :D give me more details on how to arrange the ferrites and how to do a more careful decoupling..

i don't have problem re designing the PCB layout (since it helps me get improved) but atleast if i have to develop a new PCB it should work this time :)


btw i don't see any comments on my design...
 

ericgibbs

Well-Known Member
Most Helpful Member
hi whizz

yea! i know about the ground.. but it was a messy simulation anyway... i couldn't find the right components and the appropriate values...Ltspace can't find the LM317 so your simulation also can't work correct. can you help on that?

Place these files in the appropriate LTS folders.:)

Note: on LTspice Yahoo site is a Electromet mike model.

The uploader will not accept asy and sub extensions, so change the

lm317.asm to lm317.sub and lm317.txt to lm317.asy
 

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marcbarker

New Member
where is the upstream? :D give me more details on how to arrange the ferrites and how to do a more careful decoupling..

i don't have problem re designing the PCB layout (since it helps me get improved) but atleast if i have to develop a new PCB it should work this time :)


btw i don't see any comments on my design...

Before add ferrites I think you ought to temporarily stop the oscillator and see if it's RF that's making the LDO malfunction.

By upstream of 'something', I mean at the power supply line that feeds the 'something'. Upstream as in water flowing in a river. I hope you know what I mean.

I would put a ferrite in the power line feeding the oscillator. The ferrite to attenuate the RF reaching the LDO.

I'm puzzled why there's surface mount components used on your PCB? I am sure leaded components work just as well and they are easier to use.
You know I normally recommend people to do as much as they can to design an unfamiliar RF circuit first, before doing a PCB for it. If it's leaded components, you can even develop this circuit on 2 short lengths of tinned copper wire screwed to a peice of wood! Then when you're happy with the design, you can do a PCB. Some people I know prototype the PCB layout on blank sheet with tinned copper wire as though it is etched tracks.
 
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whiz115

Member
Before add ferrites I think you ought to temporarily stop the oscillator and see if it's RF that's making the LDO malfunction.

i did it... but usualy the voltage is steady at 3.28V (since i added the ferrite bead i've noticed it to be more steady) so even if i stop the oscillator.. i can't know if something is wrong while the voltage stays steady.

I would put a ferrite in the power line feeding the oscillator. The ferrite to attenuate the RF reaching the LDO.

ok i'll do that... i'll add one more ferrite at the regulator output.

I'm puzzled why there's surface mount components used on your PCB? I am sure leaded components work just as well and they are easier to use.
You know I normally recommend people to do as much as they can to design an unfamiliar RF circuit first, before doing a PCB for it. If it's leaded components, you can even develop this circuit on 2 short lengths of tinned copper wire screwed to a peice of wood! Then when you're happy with the design, you can do a PCB. Some people I know prototype the PCB layout on blank sheet with tinned copper wire as though it is etched tracks.

this is my first design using SMD components i'm very new on PCB designing.

the general idea of the circuit it is suppose to be tested and working long before i learned it's existance... :D also a year ago i have done a thru hole version on a strip board (excluding the regulated power supply and the second transistor) which kinda worked...but the frequency was rolling all the time and it wasn't useful to play with.

Now i'm taking that old very basic circuit and i'm improving it with the ideas taken from Audioguru design.

but as a project has no use other than the educational part, though it gets very dissapointing if you can't make it work.
 

whiz115

Member
hi

what's the purpose of these links? taking ideas on how to improve my FM transmitter?

so far done nothing...

i only know the oscillator works... since i checked it using Audioguru's RF field strength meter... though i see nothing on the frequency meter... while other older versions of this transmitter which they are missing the final stage with the buffer transistor are detectable by the frequency meter.

i got an LM2940-5 i plan to use it so i can see if the power supply gets more steady without any oscillations...but i'm starting to get borred a bit...

i wouldn't expect to be so mess tunning that circuit.
 
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