How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

"newbe_1957" wrote in message
om...
How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

It's a good idea to choose an IF that is well away from any strong signals.
If you are making your own ladder filter you also need to choose a frequency
for which cheap crystals are readily available.

73, Leon

"newbe_1957" wrote in message
om...
How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

It's a good idea to choose an IF that is well away from any strong signals.
If you are making your own ladder filter you also need to choose a frequency
for which cheap crystals are readily available.

73, Leon

First decide if it's going be to single or double conversion. Easiest woulb
be single with 455 KHz, readily available IF cans, filters and xtals. If
you go double conversion you need to decide if you going to a relatively
high IF such as 40 or 60 MHz but for starters 10.7 mhZwould be good as parts
are easier to get. Read the ARRL handbook.
Good luck
73
Hank WD5JFR
"newbe_1957" wrote in message
om...
How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

First decide if it's going be to single or double conversion. Easiest woulb
be single with 455 KHz, readily available IF cans, filters and xtals. If
you go double conversion you need to decide if you going to a relatively
high IF such as 40 or 60 MHz but for starters 10.7 mhZwould be good as parts
are easier to get. Read the ARRL handbook.
Good luck
73
Hank WD5JFR
"newbe_1957" wrote in message
om...
How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

You have a number of constraints here. Deciding to cover all the HF bands
makes it particularly messy. You might consider building a few single
banders to get a feel for the problem space.

As Leon said, you want to choose an IF that is well away from any strong
signals.

However, you would also like the IF to be well away from both the received
signal and the VFO. This helps with image rejection. This is the reason
many commercial receivers have a first IF up around 60 MHz. In amateur
homebrew practice, you often try to place the IF roughly midway between the
received signal and the VFO. If you want to cover all the bands this isn't
possible.

As Hank mentioned, selecting a "common" IF means that you can get crystal
filters, transformers and the like. However, most of the stuff for common
IF frequencies is intended for broadcast use, and is too wide to be useful
for amateur practice, so be sure you know you can get what you want. 455 is
nice because you can get nice Collins filters, but this almost forces you to
double or triple conversion. A low IF makes it easier to build sharp
filters. As Leon pointed out, you want to be able to get crystals for your
selected IF very cheaply, assuming you will build the filter. You will want
to select the best handful from a big bunch. Should you run across a deal
on 100 5 MHz crystals then maybe you should pick 5 MHz.

I disagree with Hank on the 10.7 choice. 10.7 MHz parts suitable for
amateur HF service are virtually impossible to get, and you probably have a
dozen 10.7 MHz generating devices in your house. However, if you are
building an FM Broadcast receiver 10.7 MHz parts are widely available. 455
is nice because sharp filters are available, but that is really only
feasible in a double or triple conversion setup. There is no way you are
going to eliminate images with a single conversion 455 KHz IF for the higher
frequencies.

Finally, if you intend to use an analog VFO (which I recommend against) you
want to pick a frequency that allows for a low frequency VFO. Stabilizing a
VFO is a major pain, and it gets worse as you go up in frequency. You might
choose to build a stable VFO for, say, the 3 MHz range, then mix it with
crystal oscillators for each band. You do end up with a real challenge
keeping the images under control, however. And on 10 meters you would need
to have several bands because it's tough to get an oscillator to spread over
a large range relative to it's frequency and keep it stable.

You can see this gets to be a huge math problem, constrained by what you can
get in the way of filters or crystals to build filters. Your job is
simplified somewhat by choosing a DDS approach rather than an analog VFO,
but you still need to sort out how to avoid images in a large number of
VFO/IF/received frequency combinations.

Double or triple conversion simplifies this problem, but makes the whole rig
a lot more complex. A high first IF solves a lot of problems, but it pretty
much forces you to a DDS. A 60 MHz IF gives you a 56 (or 64) MHz VFO for 75
meters - there is no way you are going to build a 56 MHz analog oscillator
that is sufficiently stable.

I have to ask why? Building a rig to cover all the bands is a nice
technical challenge, but few folks work all, or even most, of the bands.
Most people find a few that suits their operating preferences and largely
stick to them. Also, you shouldn't imagine that building something will save
you any money. You simply don't have the buying power of an Icom or
Kenwood. On the other hand, there is nothing quite like operating your own
rig, that you designed! But you can get that thrill a lot easier with a
single band choice.

...

"newbe_1957" wrote in message
om...
How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

You have a number of constraints here. Deciding to cover all the HF bands
makes it particularly messy. You might consider building a few single
banders to get a feel for the problem space.

As Leon said, you want to choose an IF that is well away from any strong
signals.

However, you would also like the IF to be well away from both the received
signal and the VFO. This helps with image rejection. This is the reason
many commercial receivers have a first IF up around 60 MHz. In amateur
homebrew practice, you often try to place the IF roughly midway between the
received signal and the VFO. If you want to cover all the bands this isn't
possible.

As Hank mentioned, selecting a "common" IF means that you can get crystal
filters, transformers and the like. However, most of the stuff for common
IF frequencies is intended for broadcast use, and is too wide to be useful
for amateur practice, so be sure you know you can get what you want. 455 is
nice because you can get nice Collins filters, but this almost forces you to
double or triple conversion. A low IF makes it easier to build sharp
filters. As Leon pointed out, you want to be able to get crystals for your
selected IF very cheaply, assuming you will build the filter. You will want
to select the best handful from a big bunch. Should you run across a deal
on 100 5 MHz crystals then maybe you should pick 5 MHz.

I disagree with Hank on the 10.7 choice. 10.7 MHz parts suitable for
amateur HF service are virtually impossible to get, and you probably have a
dozen 10.7 MHz generating devices in your house. However, if you are
building an FM Broadcast receiver 10.7 MHz parts are widely available. 455
is nice because sharp filters are available, but that is really only
feasible in a double or triple conversion setup. There is no way you are
going to eliminate images with a single conversion 455 KHz IF for the higher
frequencies.

Finally, if you intend to use an analog VFO (which I recommend against) you
want to pick a frequency that allows for a low frequency VFO. Stabilizing a
VFO is a major pain, and it gets worse as you go up in frequency. You might
choose to build a stable VFO for, say, the 3 MHz range, then mix it with
crystal oscillators for each band. You do end up with a real challenge
keeping the images under control, however. And on 10 meters you would need
to have several bands because it's tough to get an oscillator to spread over
a large range relative to it's frequency and keep it stable.

You can see this gets to be a huge math problem, constrained by what you can
get in the way of filters or crystals to build filters. Your job is
simplified somewhat by choosing a DDS approach rather than an analog VFO,
but you still need to sort out how to avoid images in a large number of
VFO/IF/received frequency combinations.

Double or triple conversion simplifies this problem, but makes the whole rig
a lot more complex. A high first IF solves a lot of problems, but it pretty
much forces you to a DDS. A 60 MHz IF gives you a 56 (or 64) MHz VFO for 75
meters - there is no way you are going to build a 56 MHz analog oscillator
that is sufficiently stable.

I have to ask why? Building a rig to cover all the bands is a nice
technical challenge, but few folks work all, or even most, of the bands.
Most people find a few that suits their operating preferences and largely
stick to them. Also, you shouldn't imagine that building something will save
you any money. You simply don't have the buying power of an Icom or
Kenwood. On the other hand, there is nothing quite like operating your own
rig, that you designed! But you can get that thrill a lot easier with a
single band choice.

...

"newbe_1957" wrote in message
om...
How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

newbe_1957 wrote:

How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

Yes to all the OP, but you also need to pay attention to spurious
responses caused by mixing harmonics of the LO with harmonics of the RF.
The '76 ARRL handbook has a nice nomograph for this; I devised one for
my own use but it would take a 5000 word article to explain it.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

newbe_1957 wrote:

How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

Yes to all the OP, but you also need to pay attention to spurious
responses caused by mixing harmonics of the LO with harmonics of the RF.
The '76 ARRL handbook has a nice nomograph for this; I devised one for
my own use but it would take a 5000 word article to explain it.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi,

How do you choose a I.F. when homebrewing a H.F. receiver with BFO? I
want to build a receiver that covers all H. F. amateur bands. I am a
little confused on how to pick the type of crystal filters to use.
thanks

First catch your rabbit (i.e. a filter).

That is how a certain rabbit pie recipe starts and the same is
often true of radio projects. Joe Ham comes back from the yard sale
with a pair of tubes or a large transformer and then builds a linear
around what he has found. So go look for a good SSB/AM filter and
design the rest to suite.

A good compromise is 9MHz, as you can probably get away with
single conversion. Some of the tighter spec filters are at 455kHz
however but then you will have to go the dual-conversion route. Both
work, it is simply 'horses for courses'. If you want to put a roofing
filter up front, you could try a narrow 10.7MHz from an old mobile
radio but you will still need a narrow one after it.


73 de Joe, G3LLV