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sofiehem44

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Need help from experts!
Criteria are as follows:
A. sensitivity 0.1 uV.
B. ASK is received, (amplitude shift keying).
C. frequency: 15 MHz. (not to be used in USA).
D. type RS232 in TTL level.
When ASK is now used, carrier = 1, no carrier = 0.
I'm using Multisim 13 and have run into a wall.
So far not easy to solve.

It is more than a bit difficult to divine what kind of receiver you're talking about or the wall you have run into. What you have provided so far is not very helpful. Use your imagination and try to figure out what you might need to provide so that we can help you.

Connection ANT.enna, 0.1 uV.
NPN , BFS17P used in test.
Have tried with different circuit solutions.
I am absolutely not satisfied with the amplification of the circuit.
An expert could be an option to solve this problem.

It has a gain of over 50 - that's pretty good for an RF stage.

It may be better if you tune L2 to 15MHz and connect the collector to a tap on the coil, rather than the end?

No Antenna impedance ?
Looks like an infinite Q oscillator tank circuit.

It has a gain of over 50 - that's pretty good for an RF stage.

It may be better if you tune L2 to 15MHz and connect the collector to a tap on the coil, rather than the end?
I'm no expert, have tried my hand, not sure how simulation should be done with partitioned coils.

No Antenna impedance ?
Looks like an infinite Q oscillator tank circuit.
XF01 , has sine wave with 0.1 microvolt, see image/drawing above.

Make XF01 = 0
L2 is already tuned with C3,C4 to resonate @ 15 MHz.
See if it still oscillates in steady state. (Colpitt's Osc.)

L2 is already tuned with C3,C4 to resonate @ 15 MHz.
Without RF bypassing from the positive power to ground, it's not much of a tuned circuit.
It may work in simulation but not well in reality?

Make XF01 = 0
L2 is already tuned with C3,C4 to resonate @ 15 MHz.
See if it still oscillates in steady state. (Colpitt's Osc.)
Result:
V=0
V(p-p)=0 , I(p-p)=0
Freq=0

Without RF bypassing from the positive power to ground, it's not much of a tuned circuit.
It may work in simulation but not well in reality?
Without RF bypassing from the positive power to ground, it's not much of a tuned circuit.
It may work in simulation but not well in reality?
I actually don't know how big the sine wave should be before it will pass a bandpass filter.

I actually don't know how big the sine wave should be before it will pass a bandpass filter.
Any amplitude, if the frequency is within the passband of the filter.

The first thing the antenna signal goes to on most normal receivers is a tuned circuit or bandpass filter!

Poor Post, removed.

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I'd still like to see some decoupling capacitors, and with the extra 1 ohm added I'd like to see them either side of that.

Note how with the source to ground the circuit starts oscillating with extremely long decay time in number of cycles at 15 MHz. The Rce leakage is the only load which implies a very high Q and narrow BW. Not really practical.

same cct. with 100k load https://tinyurl.com/yu9psglx Impedance of L at 15MHz = 50 Ohms

Q = 100k/ Z(L) = 100k/50 = 2002 which is unrealistic Q value. and even more unrealistic without load=100k. This is almost a Colpitt's Oscillator with a weak base current but no C-E regeneration.

This is not how one would design a LNA 15 MHz Rx.
You must define antenna impedance and signal BW.

TI suggests a SoC solution. https://www.ti.com/lit/ml/slap127/slap127.pdf for higher f or https://www.ti.com/lit/ds/symlink/cc1201.pdf

or RFID Rx https://www.ti.com/lit/ds/symlink/trf7964a.pdf

Matching antenna impedance for low return loss with bandpass and bandstop requirements is the 1st step of a good 15 MHz Rx. Then a low noise front end amp with gain.

It may look something like this after you define better design specs for bandstop and bandpass..

Assuming this is not what you want, then I would look at how short-wave radio Rx are designed with FET front end for low noise. https://www.pinterest.ca/pin/813462751453139535/

Last edited:
same cct. with 100k load https://tinyurl.com/yu9psglx Impedance of L at 15MHz = 50 Ohms

Q = 100k/ Z(L) = 100k/50 = 2002 which is unrealistic Q value. and even more unrealistic without load=100k. This is almost a Colpitt's Oscillator with a weak base current but no C-E regeneration.

This is not how one would design a LNA 15 MHz Rx.
You must define antenna impedance and signal BW.

TI suggests a SoC solution. https://www.ti.com/lit/ml/slap127/slap127.pdf for higher f or https://www.ti.com/lit/ds/symlink/cc1201.pdf

or RFID Rx https://www.ti.com/lit/ds/symlink/trf7964a.pdf

Matching antenna impedance for low return loss with bandpass and bandstop requirements is the 1st step of a good 15 MHz Rx. Then a low noise front end amp with gain.

It may look something like this after you define better design specs for bandstop and bandpass..
View attachment 143491

View attachment 143490

Assuming this is not what you want, then I would look at how short-wave radio Rx are designed with FET front end for low noise. https://www.pinterest.ca/pin/813462751453139535/
Many thanks for your suggestions, I will look into this. Antenna can only be 1/8 wavelength, must be on a vehicle.(approx. 2.5 meters) . I have not looked for what impedance such an antenna might have. Because one of the four criteria includes frequency which is 15.0 MHz (not 14.95 or 15.05 MHz), ASK must be used, when bit is high - send carrier, when bit is low - send no/low carrier. Right now I'm trying different connections with JFET_N.
Have a nice day!

Changed conditions:- 40.0MHz- Antenna: C1,L1,R2.- Bandpass filter: C18,L6,C23,L7,L9,C20.something strange happens here if the band is too tight, the amplitude grows to over 20 Volts!!!!
- Q1: how to calculate antenna?
- Q2: where should one place frequency trimmers, how/what.
Important!! I want the carrier wave to be 5 Volts, because that's the only one I'm interested in, it's not a radio or anything like that.
Grateful for constructive suggestions, Thanks.

Firstly, forget the 1/8th wave antenna - it cannot work, it will be resonant at 80MHz.

To reduce antenna size, you add a loading coil either at the base or part way up the antenna, to make the overall item resonant at the appropriate frequency.

You can buy commercial ones:

Or make them yourself - here is a calculator to work out dimensions:

Secondly, look at proper radio receiver designs - the collector loads should be parallel resonant tuned circuits, not just chokes.

Also you need a bandpass filter (tuned circuit) between the antenna and the first transistor, otherwise that can be overloaded by any signal at any frequency.

I've not clue what that jumble of bits in the middle of the diagram is supposed to be!

Add resistors or chokes in the positive power connection between low level and high level stages, with each stage having it's own decoupling caps.

You have something like that on the collectors - but the bases are still fed directly from the non-decoupled positive power, so can pick up stray signals.

This gives a reasonable example of the first stage, the "front end" of the receiver; it's actually a converter so ignore the IC and later parts - but the first stage & RF bandpass filtering, plus the power decoupling, are good examples of what you should be aiming for.

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