Here's an interesting document regarding impedance matching networks to feed the 602/612 mixers:?

?
Page 3 tells us that the SA602A has the same mixer as the SA605, as described in this app note.
Page 3 includes some other history, but never quite says what an SA612A is (only what the SA612 was).
Page 6 shows that when driven single ended at 10mhz, the input impedance is around 5k
Page 9 reminds us that when driving the mixer differentially, that input impedance is doubled to 10k.

Page 4 of the datasheet: ??
says ? "AC input impedance is approximately 1.5k || 3 pF through 50 MHz."
I'm pretty sure that would be doubled to 3k when driven differentially.
Since this is a later document than AN1994 and is specific to the currently available SA612A,
seems more likely to be correct with regard to the input impedance. ?But that's by no means certain.

This blog posts suggests that for all parts currently available, the 602's are the same as the 612's.
The story conflicts a bit with AN1994 page 3 in that it claims the upgraded part was called the 612, not the 602A.
Never says anything about the A variants, though that transition happened around 1990, long before this was written.
Also explains that the NE612 was good only from 0-70C, whereas the SE612 is good from -40 to 85C.
The only 602/612 part I see being sold these days is the SA612AD.

If you don't need to wring every ounce of gain from these mixers
should be ok to drive them from a lower impedance source.
Some successful QRP rigs just drive them from 50 ohms single ended,
as atmospheric noise tends to predominate at HF anyway.


Jerry, KE7ER

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Oh WOW!? What a wonderful sight.? They are going to be working from now util Next Christmas!

They really look nice!

Congratulations Ash

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

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I would start out differently?? Estimate what the inductance and core (ball park) should be first.

The capacity seems pretty high, old AM radios used 365 pF down at a mHz ( I am old, so I used them alot)

And 1.5 uH is pretty small.

Take and determine your secondary winding first.? Since it is 1500 ohms, make your XL that for starts.

On a broad band inductor you go several time higher.

Goto Toriod King and use their calculator.?? Winding lots of turns is not fun and could be probematic.

Find something about 50 T less or more.? Higher permeability cores make more inductance per turn, but will

have more variability.? I have been using -6 and some -2.?? I have IF Amp design using 4 to 9uh using 3/8 and 1/2" -6.

Let us know what you pick!

Mike, WA6ISP

?

Since the very helpful reply I received from Jean-Matthieu about the Le Tenor radio I have been searching for information about the tuned circuit.

I have read that this type of circuit is a double tuned circuit and so have been reading a lot about this. I think I see how they work so am just going to outline my ideas below in case I am missing something:

Initial data:

input impedance (from antenna): 50 ohms
input impedance to NE602 (1500 ohms)
required tuned frequency: 7MHz
toroid used: T50-2

calculations:

start by picking a capacitor: 280pF (chosen as I have seen this value used in several circuits)
inductor needed to resonate a parallel LC circuit at 7MHz: 1.8uH
turns needed on a T50-2 for 1.8uH: 19

calculate primary turns required to match a 50 ohm antenna to this tuned circuit:

I have 50 ohms on the primary, 17 turns on the secondary and need 1500 ohms later on so try these values first so get: 3 turns on primary

Using the values from above for the second transformer I can now say I require 1500 ohms on the primary of the second toroid and need 1500 ohms for the NE602 so I can just do a 1:1 ratio for the turns on the second toroid, so 17 turns each side.

As far as I have seen (at an initial basic level) this seems to be a good start.

(using Jean-Matthieu's windings of 3 on the primary and 25 on the secondary on a T50-2 for both toroids with 50 ohms primary impedance, I calculate a secondary impedance of 3472 ohms. As I need this matched to 1500 on the NE602 I then calculate I need 16 turns on the secondary of the second toroid to make this match when saying I have 3472 ohms on the primary side)

I do have one more question that I have been searching for the answer to - when impedance matching from say 50 ohms into the tuned circuit is it better to go for a higher value for the reflected impedance part, above I have chosen to use 1500 ohms but Jean-Matthieu calculations suggest 3472 ohms?

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

Me too ... how to order? Nothing on hfsigs yet!?

Hi
thank you, a 1k.-resistor did the trick !

73 Gert DG3OQ

Since the very helpful reply I received from Jean-Matthieu about the Le Tenor radio I have been searching for information about the tuned circuit.

I have read that this type of circuit is a double tuned circuit and so have been reading a lot about this. I think I see how they work so am just going to outline my ideas below in case I am missing something:

Initial data:

input impedance (from antenna): 50 ohms
input impedance to NE602 (1500 ohms)
required tuned frequency: 7MHz
toroid used: T50-2

calculations:

start by picking a capacitor: 280pF (chosen as I have seen this value used in several circuits)
inductor needed to resonate a parallel LC circuit at 7MHz: 1.8uH
turns needed on a T50-2 for 1.8uH: 19

calculate primary turns required to match a 50 ohm antenna to this tuned circuit:

I have 50 ohms on the primary, 17 turns on the secondary and need 1500 ohms later on so try these values first so get: 3 turns on primary

Using the values from above for the second transformer I can now say I require 1500 ohms on the primary of the second toroid and need 1500 ohms for the NE602 so I can just do a 1:1 ratio for the turns on the second toroid, so 17 turns each side.

As far as I have seen (at an initial basic level) this seems to be a good start.

(using Jean-Matthieu's windings of 3 on the primary and 25 on the secondary on a T50-2 for both toroids with 50 ohms primary impedance, I calculate a secondary impedance of 3472 ohms. As I need this matched to 1500 on the NE602 I then calculate I need 16 turns on the secondary of the second toroid to make this match when saying I have 3472 ohms on the primary side)

I do have one more question that I have been searching for the answer to - when impedance matching from say 50 ohms into the tuned circuit is it better to go for a higher value for the reflected impedance part, above I have chosen to use 1500 ohms but Jean-Matthieu calculations suggest 3472 ohms?

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Thanks Joe and Michael,
Ed Kelley
828 Althauser St.
Dubuque, IA 52001

PS send me your PayPal email. Thanks
Ed

Ok, update: ?Michael has decided to pass, so Ed looks like you’re next in line. Just let me know where to send it. ?

First: I take it the empty pads etc have markings? D an VR numbers? If so, they're most likely superseded by? a 'double-diode package. The original diodes were iN4148(equivalent)s and the trimpot was to balance them. The new package is twinned Schottkys (far better low-signal performance), and manufactured on the one lump of silicon and so inherently balanced. The C103 trimmer was provided *for* as an option to adjust for better audio by pulling the BFO's Xtal frequency a little to a better position relative to the edge of the Xtal Filter's slope (they all can vary a little with manufacturing tolerances). Search back in the Group's Message archive for more in-depth explanation, but upshot is, as has been emphasised here often,"It's a work in progress". Note that yours is V3 - there may yet be a V4 - that's up to Farhan VU2ESE, its creator.
This ought to be a 'sticky' on the Group's homepage ... :)

73 de ZL2DEX

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Have you worked out pricing for the Lotsa Clock pcb Mike and how did your DX mail weighing of the 2 pcb offer go?

Bill, VK7MX

Boy that was fast ?. Ok Michael, you asked first so I guess you win. Just let me know where to send it.?

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I can PayPal you say 20 to cover shipping
Ed W0IC

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I have a Hendricks Bitx17A kit and I would like a 20 to fix
Ed W0OIC
ekelley828 at centurylink.net

If you haven't had a taker I'll go for it.? ?I have some equipment that I can troubleshoot with.?