# IF Amplifier input impedance value ???

S

#### sparks

Jan 1, 1970
0
Hi,
I wonder if anyone can work out the input impedance of the 1st IF Amp
in the circuit Ive linked to below.
The frequency of the IF is 1.4MHz and Im trying to work out what the
termination impedance is of the crystal filters. The filters are SEI
brand made in the 1960's. Ive also been trying to locate some data on
the filters used, but its not getting to any facts.
I have stored a jpeg of the circuit here ....
http://www.box.net/shared/8zrb3embil

Any comments on this would be greatly apreciated. Ive tried to use
Thevenins theorem on this, but the combination of AC and DC feedback
from collector to base is beyond me in terms of what it does to poor
old Thevenin.
regards,
JEFF
ZL3JK
New Zealand

P

#### Phil Allison

Jan 1, 1970
0
"sparks"
Hi,
I wonder if anyone can work out the input impedance of the 1st IF Amp
in the circuit Ive linked to below.
The frequency of the IF is 1.4MHz and Im trying to work out what the
termination impedance is of the crystal filters. The filters are SEI
brand made in the 1960's. Ive also been trying to locate some data on
the filters used, but its not getting to any facts.
I have stored a jpeg of the circuit here ....
http://www.box.net/shared/8zrb3embil

Any comments on this would be greatly apreciated. Ive tried to use
Thevenins theorem on this, but the combination of AC and DC feedback
from collector to base is beyond me in terms of what it does to poor
old Thevenin.

** I reckon there is very little IF signal appearing across R57 (150 ohms)
as the primary Z of T13 is much higher. So, the input Z is simply R54 plus
R57 ( ie 300ohms) in parallel with the input Z at the base of VT13.

( VT13 = RCA 40235, Hfe =170, Ft = 1GHz )

But VT13 has an unbypassed emitter resistor of R58 (27 ohms), so its base Z
is many times higher.

So, the load as seen by the Xtal filters is fairly close to 300 ohms, say
250 ohms.

Seems a not unlikely value for Xtal filters.

...... Phil

A

#### Andrew Holme

Jan 1, 1970
0
sparks said:
Hi,
I wonder if anyone can work out the input impedance of the 1st IF Amp
in the circuit Ive linked to below.
The frequency of the IF is 1.4MHz and Im trying to work out what the
termination impedance is of the crystal filters. The filters are SEI
brand made in the 1960's. Ive also been trying to locate some data on
the filters used, but its not getting to any facts.
I have stored a jpeg of the circuit here ....
http://www.box.net/shared/8zrb3embil

Any comments on this would be greatly apreciated. Ive tried to use
Thevenins theorem on this, but the combination of AC and DC feedback
from collector to base is beyond me in terms of what it does to poor
old Thevenin.
regards,
JEFF
ZL3JK
New Zealand

View in a fixed pitch font:

150 150
Vin Iin ___ V2 I2 ___
-------->---|___|----o--->--|___|----.
| |
| |
/ \ |
( | ) Ic=Vin/27 |
\V/ |
| |
| |
=== ===
GND GND

Iin = Ic + I2 = Vin/27 + I2 = Vin/27 + V2/150

I2 = V2/150 = (Vin - 150*In)/150

2*Iin = Vin/27 + Vin/150

Vin/In ~ 50 Ohms

The LTSpice simulation below seems to agree with this.

Do those filters contain internal matching transformers?

Version 4
SHEET 1 880 680
WIRE -80 -32 -224 -32
WIRE 112 -32 -16 -32
WIRE 288 -32 112 -32
WIRE 448 -32 288 -32
WIRE 688 -32 528 -32
WIRE 288 0 288 -32
WIRE -224 16 -224 -32
WIRE 112 16 112 -32
WIRE 688 16 688 -32
WIRE 288 112 288 80
WIRE -224 160 -224 96
WIRE -96 160 -224 160
WIRE 48 160 -96 160
WIRE 112 160 112 96
WIRE 112 160 48 160
WIRE 224 160 112 160
WIRE 688 160 688 96
WIRE 288 240 288 208
WIRE 48 288 48 160
WIRE 656 336 496 336
WIRE -224 352 -224 160
WIRE 288 352 288 320
WIRE 288 352 160 352
WIRE 496 368 496 336
WIRE 656 368 656 336
WIRE 288 400 288 352
WIRE 160 416 160 352
WIRE 496 480 496 448
WIRE 576 480 496 480
WIRE 656 480 656 448
WIRE 656 480 576 480
WIRE -224 528 -224 432
WIRE 48 528 48 368
WIRE 160 528 160 480
WIRE 288 528 288 480
WIRE 576 528 576 480
FLAG -224 528 0
FLAG 48 528 0
FLAG 288 528 0
FLAG 688 160 0
FLAG 160 528 0
FLAG 576 528 0
FLAG -96 160 Vin
SYMBOL res 640 352 R0
SYMATTR InstName R1
SYMATTR Value 4k7
SYMBOL res 544 -48 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 150
SYMBOL res 272 224 R0
SYMATTR InstName R3
SYMATTR Value 27
SYMBOL res 272 384 R0
SYMATTR InstName R4
SYMATTR Value 220
SYMBOL res 96 0 R0
SYMATTR InstName R5
SYMATTR Value 3k3
SYMBOL res -240 0 R0
SYMATTR InstName R6
SYMATTR Value 150
SYMBOL res 32 272 R0
SYMATTR InstName R7
SYMATTR Value 3k3
SYMBOL cap 144 416 R0
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL cap -16 -48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 100n
SYMBOL current -224 352 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value SINE(0 1u 1.4e6)
SYMBOL voltage 688 0 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL ind2 272 -16 R0
SYMATTR InstName L1
SYMATTR Value 5m
SYMATTR Type ind
SYMBOL ind2 480 352 R0
SYMATTR InstName L2
SYMATTR Value 5m
SYMATTR Type ind
SYMBOL npn 224 112 R0
SYMATTR InstName Q1
TEXT 406 160 Left 0 !.tran 5u
TEXT 408 88 Left 0 !K L1 L2 1

A

#### Andrew Holme

Jan 1, 1970
0
Andrew Holme said:
View in a fixed pitch font:

150 150
Vin Iin ___ V2 I2 ___
-------->---|___|----o--->--|___|----.
| |
| |
/ \ |
( | ) Ic=Vin/27 |
\V/ |
| |
| |
=== ===
GND GND

Iin = Ic + I2 = Vin/27 + I2 = Vin/27 + V2/150

I2 = V2/150 = (Vin - 150*In)/150

2*Iin = Vin/27 + Vin/150

Vin/In ~ 50 Ohms

The LTSpice simulation below seems to agree with this.

Do those filters contain internal matching transformers?

The above ignores re which is about 2 ohms, since Ie ~ 14mA.

This gives Ic = Vin/29 which gives Vin/Iin even closer to 50 ohms.

A surprising result for matching crystal filters.

S

#### sparks

Jan 1, 1970
0
Thanks for the help on this guys.
In the interim, Ive had some advice from someone who used to work at
Redifon (the radio's manufacturer) and he says the filters they used
were sometimes (but not always, depending on radio model) 50
ohms.These filters used small internal matching transformers inside
the case. There were other options however for filter value such as
280 and 680 ohms.
So it looks like the filters are 50 ohms by your calculations....
Thats great confirmation.
regards,
JEFF

S

#### sparks

Jan 1, 1970
0
Guys,
I did chop the circuit off a it too cropped. The line going down off
screen via C37 is going to an AGC diode attenuator network, so
basically the diodes only conduct when a reasonanble signal is present
to reduce the gain. Otherwise C37 is floating.
JEFF

S

#### sparks

Jan 1, 1970
0
Hi Guys,
Ive placed another circuit of the IF Amp on box.net, which contains
the missing diode atten network. Its here...
http://www.box.net/shared/hvem3ip2zx

I have another 2 related questions on the design I wonder if you could

What do I need to change in the circuit so to increase the input
impedance to 910 ohms (so I can use another more modern brand of
filter) ?
Would this alteration to 910 ohms still be consistent if the IF freq
was moved up to 10.7Mhz, as this is the preferred filter freq Id like
to use.

regards,
JEFF

A

#### Andrew Holme

Jan 1, 1970
0
sparks said:
Ive placed another circuit of the IF Amp on box.net, which contains
the missing diode atten network. Its here...
http://www.box.net/shared/hvem3ip2zx

I have another 2 related questions on the design I wonder if you could

What do I need to change in the circuit so to increase the input
impedance to 910 ohms (so I can use another more modern brand of
filter) ?

Remove R54. Reduce R55 and R56 to 1K8.
Would this alteration to 910 ohms still be consistent if the IF freq
was moved up to 10.7Mhz, as this is the preferred filter freq Id like
to use.

Should be. T13 might be less efficient at 10.7 MHz.

Are there any tuned circuits in the later IF stages? What about the BFO?
Changing IF frequency sounds a bit drastic.

A

#### Andrew Holme

Jan 1, 1970
0
Andrew Holme said:
Remove R54. Reduce R55 and R56 to 1K8.

CORRECTION:
Just tried this in simulation and it doesn't work. This is better:

Leave R55 and R56 alone.
Remove R54.
Short the cold end of C34 to ground and/or short-out R57

S

#### sparks

Jan 1, 1970
0
Thanks Andrew. Im wanting to use a wider filter and hence the change
to 10.7Mhz IF where I can obtain suitable (free) 8 pole filters of
various bandwidths.
These filters are 910 ohms, hence the question re changes required to
accommodate the 910 termination impedance.
Its probably likely that the transistors used in the IF wont be too
good at 10.7Mhz, but I havent checked their spec yet.
Yes, there is a couple of tuned circuits in the last part of the IF.
Noise shaping the manual says they are for. The first is balanced
output to accommodate a balanced crystal filter config for CW
reception, but I plan to gut it, and use only one tapped tuned circuit
to match into the detector. BFO is present but not required.
How easy / expensive is LTSpice? Ive heard of it, but never seen it in
action.
JEFF

A

#### Andrew Holme

Jan 1, 1970
0
sparks said:
Thanks Andrew. Im wanting to use a wider filter and hence the change
to 10.7Mhz IF where I can obtain suitable (free) 8 pole filters of
various bandwidths.
These filters are 910 ohms, hence the question re changes required to
accommodate the 910 termination impedance.
Its probably likely that the transistors used in the IF wont be too
good at 10.7Mhz, but I havent checked their spec yet.
Yes, there is a couple of tuned circuits in the last part of the IF.
Noise shaping the manual says they are for. The first is balanced
output to accommodate a balanced crystal filter config for CW
reception, but I plan to gut it, and use only one tapped tuned circuit
to match into the detector. BFO is present but not required.
How easy / expensive is LTSpice? Ive heard of it, but never seen it in
action.

LTSpice is easy to use and free!

40235, 2N3904 and 2N3906 should be fine at 10.7 MHz

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