Can you connect a broadband rf transformer to a receiver front end tank
circuit rather than to a resistive load? The inductance of the side of
the transformer connected to the tank circuit would be several times
the inductance in the tank circuit, and changes to the overall
inductance (parallelled inductances) of the circuit would be taken
into account. Thanks

It would have to be matched to it, of course. Combiners based on
broadband transformers are often used to feed receiver inputs, when
testing front-ends for intermodulation and so on.

Leon

gojamo wrote:
Can you connect a broadband rf transformer to a receiver front end tank
circuit rather than to a resistive load?

Yes.

The inductance of the side of
the transformer connected to the tank circuit would be several times
the inductance in the tank circuit, and changes to the overall
inductance (parallelled inductances) of the circuit would be taken
into account.

You can often make a better broadband transformer by using
low-frequency, high-permeability ferrite for core material than by using
higher-frequency, lower-permeability ferrite. This results in a winding
impedance that's high, but primarily resistive through the HF range and
above. The advantages are that a high winding impedance is relatively
easy to achieve, and it stays quite constant over a very wide frequency
range -- typically several decades -- since it's free of resonant
effects. What you have to insure, then, is that the winding impedance is
at least several times higher than the impedance seen by the circuit
it's across. If it is, the fact that it's resistive rather than
inductive is of no consequence as far as the external circuit is concerned.

Roy Lewallen, W7EL

gojamo wrote:
Can you connect a broadband rf transformer to a receiver front end tank
circuit rather than to a resistive load? The inductance of the side of
the transformer connected to the tank circuit would be several times
the inductance in the tank circuit, and changes to the overall
inductance (parallelled inductances) of the circuit would be taken
into account. Thanks

....sure you can. I think you noticed that sometimes the antenna is
connected to a tap on the inductor, thus creating an autotransformer.
The one thing to beware of is to make sure that the input impedance,
reflected through the transformer is high enough, so as not to load the
tuned circuit. it's NOT a match. If matched, the impedance of the
transformer's secondary would be equal to the tuned circuit's
impedance, thus lowering the Q of the tuned circuit to one half the
original value.
Saandy 4Z5KS

Look at is when the receiver input is tuned. The receiver input will look
resistive (for example 50 ohms). The transformer is therefore loaded by a
resistive load (as it should).

73, Steve, K9DCI




"gojamo" wrote in message
oups.com...
Can you connect a broadband rf transformer to a receiver front end tank
circuit rather than to a resistive load? The inductance of the side of
the transformer connected to the tank circuit would be several times
the inductance in the tank circuit, and changes to the overall
inductance (parallelled inductances) of the circuit would be taken
into account. Thanks

Saandy,
I respectfully disagree, in part. I believe you want to re-think this.
I know it "looks like" you are just paralleling two inductors, but the whole
thing must be considered.

There is such a thing as loaded Q which is what you will see when all is
connected up. When un-connected, you have an "un-loaded Q" situation...and
they are indeed different. The loaded Q is a very important and specific
consideration when designing filters. The loading of the external circuitry
(and resulting loaded Q) has to be taken into account when designing for a
required bandwidth.
Also, there can be a match, and should be for sensitivity, aka maximum
power transfer. Well done, the receiver input tuned circuit will appear
resistive or nearly so. The tuned circuit(s) provide impedance matching
between the antenna input (50 ohms) and the first device (usually higher Z),
as well as filtering -- two functions for the price of one.
Side-bar: When considering noise figure, there can be a
different ball game, but it is a relatively small
difference - impedance wise.
A well designed RF transformer will reflect (look like) 50 ohms at its
output (receiver input) due to the (supposedly) 50 ohm antenna/feed-line
connected to its input (antenna side).

Obviously I am assuming a 50 ohm system, to explain.

A tap is a way of impedance matching and the auto-transformer analogy is
a good way to look at it. I call this a "Mental Model" for better
understanding what is going on - it works to analyze things. Because 50
ohms is a relatively low value, tapping a coil is a way to get to a lower
impedance from the relatively high impedance at the top of the coil/tuned
circuit. Capacitive tap is another common way (they just have differing
secondary [side] effects in far-out attenuation).

Comments?
73, Steve, K,9.D;C'I




"Saandy , 4Z5KS" wrote in message
oups.com...

gojamo wrote:
Can you connect a broadband rf transformer to a receiver front end tank
circuit rather than to a resistive load? The inductance of the side of
the transformer connected to the tank circuit would be several times
the inductance in the tank circuit, and changes to the overall
inductance (parallelled inductances) of the circuit would be taken
into account. Thanks

...sure you can. I think you noticed that sometimes the antenna is
connected to a tap on the inductor, thus creating an autotransformer.
The one thing to beware of is to make sure that the input impedance,
reflected through the transformer is high enough, so as not to load the
tuned circuit. it's NOT a match. If matched, the impedance of the
transformer's secondary would be equal to the tuned circuit's
impedance, thus lowering the Q of the tuned circuit to one half the
original value.
Saandy 4Z5KS