Harros

What differs the crystal oscillator? (as i can see that there are different load capacitance in the crystal oscillator) Any comments on the amplifier that i should build for the crystal oscillator?

Harros

How to determine the value of the components used (I think i cant build the filter using the lookup table in this case...)?

RadioRon

Start with this:

http://www.allaboutcircuits.com/vol_2/chpt_6/2.html

then, with focus on parallel resonant circuit, study this:

http://www.allaboutcircuits.com/vol_2/chpt_6/6.html

then this to see how to couple into and out of a parallel resonanct circuit

http://www.allaboutcircuits.com/vol_2/chpt_8/6.html

Then build your model using these principles, plus, when you have three parallel tuned circuits, you can couple from one to the next by using a capacitor at the top of the circuit, but start with a very small value of capacitor, and then gradually increase this capacitor and see the effect on insertion loss and bandwidth.

__________________
RadioRon

Harros

Can i use the same filter with the same configuration for the preselect filter and the filter between the RF amplifier and the phase detector?

RadioRon

Yes, but you must adjust each input/output of each filter for a good impedance match.

__________________
RadioRon

Harros

Hi, I have built the BP filter with Q = 1M/70k = 15 approximately, and ~50 ohm for input/output impedance. Any comments on this filter? However the rolloff at upper frequencies seems very slow, will it influence the performance of this filter? Besides, there is a great change of phase response around 1MHz region, will it cause the phase delta measurement to be inaccurate?

Attached Images

	Simulation_Filter1.JPG (118.1 KB, 4 views)
	Simulation_Filter2.JPG (109.8 KB, 2 views)
	Simulation_Filter3.JPG (125.2 KB, 2 views)
	Simulation_Filter4.JPG (106.8 KB, 4 views)

Attached Files

Preselect Filter.pdf (36.1 KB, 5 views)

mvs sarma

please look at this,

http://knallfunke.de/drm/antennen/mobileloop/index.htm

Translation can be done by yahoo translator


All the best

__________________
Regards,
Sarma.

Harros

Thank you for your info, sarma.

RadioRon

The way that you couple the 50 ohm source directly to the input and the 50 ohm load directly to the output is severely reducing the Q of the first tank and the third tank, so the overall filter Q is not good enough. This is evidenced by your measurement of Q which is only 10. This is quite poor and I expect at least 50 and perhaps as high as 200 in the simulation. It will be lower in practice, depending on the Q that you have assigned to the coil in the sim.

Imagine that there is no source attached to the input. In that case, the loaded Q of the first tank circuit might be as high as 50 to 100 or more. However, when you attach the 50 ohm source directly to the top of L1 like that, it is identical to putting a 50 ohm resistor in parallel with L1 which dramatically reduces the loaded Q.

The fact that the rolloff is slow, and levels out, at frequencies above the center frequency of the filter is due to the same problem. As the frequency goes above 1 MHz, the impedance of the top coupling capacitors C2 and C4 goes lower plus the effect of L1, L2 and L3 disappear and so there is nothing with an increasing impedance vs frequency in the way of the energy, so the output levels out at about -50 dB.

The next step to fix these problems is to change the connection point of R1 and of R2. For example, with R1, you should disconnect it from the top of L1 and create a "tap" on L1 about 1/10 from the bottom (that is, 10% of the total number of turns of wire) and then connect R1 to that tap. Similarly, disconnect R2 from C5/L3, create a tap on L3 at the same height as you did for L1, and reconnect R2 there.

This method of tapping the coil will improve the impedance match between the 50 ohm source/load and the tank circuit, it will dramatically increase the loaded Q of the first and third tank, and it will cause the ultimate rejection above resonance to keep going down without levelling out. You may adjust the height of the tap for best overall performance, but it should be kept fairly low on the coil.

The change in phase at the resonant frequency doesn't matter. Ultimately, you will be operating at only one frequency and your circuits will cause a total phase shift of some unknown amount. It will be necessary to calibrate out this phase shift. The important thing is that the phase shift isn't varying with time, and that won't happen in this circuit.

__________________
RadioRon

Hero999

You don't even need to to tap the coil.

Another winding loosly coupled to the coil will do the job - it'll act sa a transformer. In theory you could have the loop outside your house and the coil in front of your bedroom window coupling the energy.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

Harros

By the way, I did not assign the Q of the coils during simulation, will it cause my simulation to be inaccurate? Should i do the simulation without coupling the source impedance and load impedance?

RadioRon

Your sim is probably inaccurate. What Q does the sim assume? Infinite?

How would you do the sim without a source?

__________________
RadioRon

Harros

I guess so. There is no such an option to allow me to set the Q factor of the inductor that i use in simulation. The freq response of the filter now is much better. But the bandwidth is uneven as i can see a wider band at upper freq. What should i do?

Attached Images

	filter1.JPG (118.2 KB, 7 views)
	filter2.JPG (112.4 KB, 4 views)

Attached Files

filter.pdf (37.1 KB, 3 views)

RadioRon

You should add a resistor in series with each inductor. These resistors represent the resistive losses in the coil and each coil together with each resistor becomes a better model of a real inductor. The resistor value should be calculated by dividing the inductive reactance of the coil by its Q, and you can assume a Q of 80 for this.
Next, you can tune the resonant frequency of each of the three tanks circuits to seek the most desireable response. Tuning means to vary either the inductance or capacitance value of each tank circuit a small amount and also the value of the top-coupling capacitors. What is the most desireable response? If you are interested in passing only one sinusoid, at 1.00 MHz and you know this sinusoid is stable, then you can tune your circuit for the most extremely narrow response. If you considered it wise to have a bit more bandwidth in your filter to deal with variabilities then you might choose to tune one tank to exactly 1.000, another tank to 1.001 and another to .999 MHz to flatten the response. This final tuning is up to you.

Do not be concerned with a bit of assymettry to the response as you point out. This is harmless. Also, pay no attention to the response below -50 dB because when this filter is built, there will be unexpected parasitic coupling that will limit the response and not allow it to fall below -50 or -60 dB. So, your plot showing attenuation beyond -100 dB is not realistic.

This filter is working well.

I think that it would be a lot more informative if your x axis were expanded to show performance mainly from 100Khz to 10 MHz.

__________________
RadioRon

Harros

I can observe that the quality factor for most of the inductors sold in the market are around 35 to 45, should i put the corresponding Q of the particular inductor rather than the Q of 80? I am now working on designing the amplifier, is it alright for me to use 2n3904 in the amplifier?

I am now satisfied with the filter response (please refer to the pics, and circuit)... How do you think? I made the capacitors in the CL tank to be in parallel as i cant get a component with such a exact value , is it alright for us to do so?

Again, whats the type of connector that i should use to connect all the modules?

Attached Images

	filter1.JPG (125.8 KB, 5 views)
	filter2.JPG (113.0 KB, 5 views)

Attached Files

Preselect Filter_Tank Cheby.pdf (38.5 KB, 4 views)