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 Post subject: VFO Isolation Buffer Simulation
PostPosted: Sep Mon 06, 2021 8:50 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
I got interested in so called isolation amplifier or buffer for LC tuned VFO. This particular circuit is tweaked for FM band operation but changing the component values around the first transistor will set 50ohm input impedance for your frequency of interest. No RF transistors have been used, just your garden variety 2n3904.

In simulation the input impedance stayed constant even with 1, 50, 75 ohm, and 100K resistors as output load. The output is heavily isolated from the input. Schematic and impedance graph attached. Zin varies from 57ohm at 88mhz to 49.5ohm at 108mhz but stays the same regardless of the output load (1ohm, 50ohm, 100k). In other words, the output termination has no effect on the input, the input impedance stays stiff. 1ohm and 100K load is equivalent to a short and open circuit at the output.

The three cascaded emitter followers in darlington configuration are responsible for the high reverse isolation. They not only work great at DC but also at RF! Using a few 2n3904 eliminates the need of a JFET like MPF102 or J310 for VFO buffer stage. The darlington pair does the trick.


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Sun 12, 2021 1:00 am 
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Joined: Nov Sun 22, 2015 4:51 pm
Posts: 40
Exactly what are we looking at in the graph? I see a scale in tens of volts; that circuit will not produce anything like such at its output, at any frequency. I also see two traces in the graph, but only one voltage probe response shown at the top of the graph. What are the two traces?

What are the operating points of the transistors? That is, what are the collector currents as we move down the chain? Considering the 50ish volts Y scale, are they anything like within normal for a 2N3904? Because many of the devices are dc coupled and there's no negative feedback to compensate, slight variations in device characteristics in the chain will reflect up and down the chain.

Simulation is only the beginning, and I suspect that the SPICE 2N3904 model may not be particularly realistic at VHF. Real-world aspects of components will change things. For instance, a 100-nF capacitor with any leads at all will be an inductor, not a bypass capacitor, in the FM band; likewise for a 1-nF inductor used as a coupling C. So that positive-rail bypass cap won't bring that rail down close to RF common in a real implementation -- probably even if you used surface-mount devices in an RF-fluent PC board design.

The 2N3904 has a transition frequency of 300 MHz; it's not really a VHF device. UHF devices would be better. But beware the emitter follower, of which your extended Darlington chain consists: Followers built with any device -- vacuum tube, JFET, MOSFET, BJT -- only need a little inductance in their emitter circuitry to turn into oscillators. (After all, a follower is the basis of several popular oscillator topologies; the Colpitts and Hartley oscillators come to mind.)

The proof of the pudding for oscillator buffering is what happens to the measured oscillation frequency when we play with terminations at the buffer output. An oscillator is the most sensitive indicator of changes in its environment one can find; even impedance changes that may look like math noise in a simulator will produce measurable changes in the frequency of an oscillator, especially one operating at VHF or higher.

I recommend building the circuit to see if behaves like its simulation.


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Sun 12, 2021 4:50 am 
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Joined: Mar Sun 22, 2020 5:56 am
Posts: 1995
Location: Arvada, CO, 80004
I thing it would act similar, but not exact. The leads of the pins on resistors, transistors, and whatnot add capacitance, and inductance. That will effect frequencies at such proportion.

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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Sun 12, 2021 7:42 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
W9BRD wrote:
Exactly what are we looking at in the graph? I see a scale in tens of volts; that circuit will not produce anything like such at its output, at any frequency. I also see two traces in the graph, but only one voltage probe response shown at the top of the graph. What are the two traces?

What are the operating points of the transistors? That is, what are the collector currents as we move down the chain? Considering the 50ish volts Y scale, are they anything like within normal for a 2N3904? Because many of the devices are dc coupled and there's no negative feedback to compensate, slight variations in device characteristics in the chain will reflect up and down the chain.

Simulation is only the beginning, and I suspect that the SPICE 2N3904 model may not be particularly realistic at VHF. Real-world aspects of components will change things. For instance, a 100-nF capacitor with any leads at all will be an inductor, not a bypass capacitor, in the FM band; likewise for a 1-nF inductor used as a coupling C. So that positive-rail bypass cap won't bring that rail down close to RF common in a real implementation -- probably even if you used surface-mount devices in an RF-fluent PC board design.

The 2N3904 has a transition frequency of 300 MHz; it's not really a VHF device. UHF devices would be better. But beware the emitter follower, of which your extended Darlington chain consists: Followers built with any device -- vacuum tube, JFET, MOSFET, BJT -- only need a little inductance in their emitter circuitry to turn into oscillators. (After all, a follower is the basis of several popular oscillator topologies; the Colpitts and Hartley oscillators come to mind.)

The proof of the pudding for oscillator buffering is what happens to the measured oscillation frequency when we play with terminations at the buffer output. An oscillator is the most sensitive indicator of changes in its environment one can find; even impedance changes that may look like math noise in a simulator will produce measurable changes in the frequency of an oscillator, especially one operating at VHF or higher.

I recommend building the circuit to see if behaves like its simulation.


The graph is input impedance. Ignore the Volt, it's actually Ohm. V / 1A current sweep = impedance in ohms.

2n3904 FT goes up to 330MHz. I've tested such buffers in real life. A common base stage gives the best isolation. On simulation, change output load to 1ohm, 50ohm, and 100K and monitor peak to peak voltage at the output of the oscillator. The less it changes, the more isolation there is.

Several cascaded transistors is the solution. 2n3904 rocks!!


Last edited by Dare4444 on Sep Sun 12, 2021 12:20 pm, edited 1 time in total.

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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Sun 12, 2021 7:56 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Real life observation. Oscillator frequency is stable only when power is drawn from collector with a series resistor. R12 sets input Z to 2.2K matching it to collector. Q6 is common base stage. Adjust the capacitor in parallel to L11 for max output (68pf for 98MHz). RFCs are 70T thin wire on 10k 1/4W carbon film resistor. Output is taken from collector of Q6. Showed S12 or reverse isolation of at least -50dB in spice. 2n3904 has low FT that's why several of them are used. It's a very effective buffer and eliminates drift as very little power is drawn from the oscillator. Tested in real life. Robust performance. Spice is really accurate and more than -50dB of reverse isolation at 100MHz using 2n3904 transistors! We don't need RF transistors to build a buffer with high S12.


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Sun 12, 2021 10:42 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Another weird buffer circuit to isolate the oscillator from antenna. It's meant for single transistor FM transmitters with a collector coil with one or two turn tap at the top end for antenna connection. Their frequency drifts all over the place. This buffer would stabilize such a single transistor TX and connect directly to the tap on the oscillator coil. It's a pair of 2n3906 in Darlington configuration. It then feeds two cascaded emitter followers based on 2n3904 to increase isolation between oscillator and antenna. It's a result of half an hour playing around on spice. The Darlington input interface is a bit weird.

Note: Spice simulation = input impedance is around 220ohm. A collector coil with 7T and tap at 1.5T from V+ would transform 220ohm to 4.7K at the collector. The collector would see a 4.7K impedance only (low power drawn).


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Mon 13, 2021 10:32 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1574
Sorry, it took me a while to find the image of the Opto-isolator again.

It was designed, using Honeywell photo transistor & LED, to buffer a signal from the VFO in an Eddystone EC-10 radio, export it to an output connector on the rear of the radio, in a way that meant regardless of the load on that connector, the VFO in the radio would not be pulled in frequency at all (undetectable with laboratory instruments including high resolution frequency counters).

Most multi-stage buffer circuits I tried could not achieve anywhere this degree of isolation.

I made it on a brass plate with a one square cm brass cube, with a hole through it to accommodate the LED ad the photo-transistor.

It is quite old now, probably the method, with modern devices would be good to 100MHz, I would expect.


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Mon 13, 2021 11:24 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Thank u thank u thank u. Even with very high S12 the osc is still disturbed. I'll use ur optical coupler to come up with a fm TX with only 1KHz of drift in a day!!

YOU HAVE SHOWN ME A GREAT WAY. I'LL NOW PERFECT FM TRANSMITTERS WITH NO PLL BUT HIGH STABILITY.


Last edited by Dare4444 on Sep Mon 13, 2021 1:47 pm, edited 1 time in total.

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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Mon 13, 2021 11:26 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Need ur email pls, one last buffer ckt to share. Email me I want ur opinion plus like to share. Designed today on spice. 10 hours of work trying every combination.

Joy226010 at gmail


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Mon 13, 2021 2:32 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
I'll try to do it with LEDs. They can sense light.
LiFi also uses leds


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Tue 14, 2021 7:49 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1574
Dare4444 wrote:
Need ur email pls, one last buffer ckt to share. Email me I want ur opinion plus like to share. Designed today on spice. 10 hours of work trying every combination.

Joy226010 at gmail


It is better to post the circuits here as sometimes I'm not sure how to make a remark on simulated circuits and other members with the simulator might be better making remarks on it as they could enter your circuit and tweak the parameters to see the effects. I generally design the old fashioned way with the formulae and a calculator, which can be quite time consuming for a circuit with more than a few stages.


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Tue 14, 2021 8:27 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
ACORNVALVE wrote:
Dare4444 wrote:
Need ur email pls, one last buffer ckt to share. Email me I want ur opinion plus like to share. Designed today on spice. 10 hours of work trying every combination.

Joy226010 at gmail


It is better to post the circuits here as sometimes I'm not sure how to make a remark on simulated circuits and other members with the simulator might be better making remarks on it as they could enter your circuit and tweak the parameters to see the effects. I generally design the old fashioned way with the formulae and a calculator, which can be quite time consuming for a circuit with more than a few stages.


I used to until last year. Hours and hours of work for weeks to perfect a circuit for frequency drift, etc. Then some one introduced me to Spice. I did numerous simulations and each time I built in real life the simulations matched. Ltspice has an uncanny ability to correctly predict everything. Once I was sure that spice is foolproof I began designing circuits on spice only. Instead of weeks it took a few hours! So much time and energy saved. The accuracy of spice is Unbelievable. No wonder it's the industry standard for RF ckts. Hams love it.

Ok I'll modify the circuit for lower power and post it here.


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Tue 14, 2021 11:07 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
ACORNVALVE wrote:
It is better to post the circuits here as sometimes I'm not sure how to make a remark on simulated circuits and other members with the simulator might be better making remarks on it as they could enter your circuit and tweak the parameters to see the effects. I generally design the old fashioned way with the formulae and a calculator, which can be quite time consuming for a circuit with more than a few stages.


Check private msg


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 Post subject: Re: VFO Isolation Buffer Simulation
PostPosted: Sep Tue 14, 2021 6:44 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Here's the two transistor buffer consisting of 2n3906.
The emitter of FM oscillator Q1 is directly feeding the first common base buffer stage. As the output is taken from emitter, it further isolates the LC tuned tank circuit. The third transistor is common emitter amplifier to bring the signal to 1mW level. Single transistor FM transmitter circuits drift like crazy but here it's successfully shown that adding just two extra transistors can stabilize frequency drift and completely isolate the LC tank circuit from antenna. Output is taken from collector of Q4. It's range should be around 50 feet and one can keep it running all day with no drift thanks to the buffer circuit. Just some food for thought. I know many novices visit this site who are interested in homebrew but frustrated by drifting frequency of single transistor FM transmitters. This circuit uses only a few extra parts and gives a lot of advantage. The oscillator caps should be NP0 type.


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