what does insertion/return loss mean when talking about baluns?

[Michael]

Hi - I'm rather RF handicapped so please bear with me . I'm looking
at the basic application schematic for an Atmel 802.15.4 transceiver,
the AT86RF212 (page 10: http://www.atmel.com/dyn/resources/prod_documents/doc8168.pdf).
They show the use of a balun on the differential antenna pins. I was
not aware of baluns until I saw them used here. Anyways, I took a look
at one of the recommended baluns, the JTI 0900BL18B100E (datasheet:
http://www.johansontechnology.com/products/rfc/blct/JTI_Balun-0900BL18B100_9-04.pdf),
and got a bit confused. When it refers to insertion loss and return
loss, what exactly does that mean?

Also - when trying to figure out a link budget, how do all the
passives between the IC and the antenna affect the budget?

Thanks!

-Michael

 

[RFI-EMI-GUY]

Hi - I'm rather RF handicapped so please bear with me . I'm looking
at the basic application schematic for an Atmel 802.15.4 transceiver,
the AT86RF212 (page 10: http://www.atmel.com/dyn/resources/prod_documents/doc8168.pdf).
They show the use of a balun on the differential antenna pins. I was
not aware of baluns until I saw them used here. Anyways, I took a look
at one of the recommended baluns, the JTI 0900BL18B100E (datasheet:
http://www.johansontechnology.com/products/rfc/blct/JTI_Balun-0900BL18B100_9-04.pdf),
and got a bit confused. When it refers to insertion loss and return
loss, what exactly does that mean?

Also - when trying to figure out a link budget, how do all the
passives between the IC and the antenna affect the budget?

Thanks!

-Michael

Insertion loss is expressed in dB and pertains to the efficiency of the
balun when passing a signal. A balun with a 1.2 dB insertion loss is
somewhat less efficient than one with 0.2 dB IL. For example if a device
has a 3 dB insertion loss and you generate 100 watts (+50 dBm) at its
input you will lose 1/2 the power and will have only 50 watts (+47 dBm)
at the output. Additionally the device has to dissipate the lost energy
as heat and this will also affect its longevity, unless it is power
rated accordingly.

The insertion loss specification assumes that the device is properly
terminated, bringing us to the next question:

Return loss pertains to the power reflected back from a device at its
input (and likewise output) port. Return loss is measured with a return
loss bridge or other device. This is a figure of merit of the proper
matching of the source impedance to the load impedance.

Imagine a spot light focused on a mirror, nearly all of the power will
be reflected back to the source. Alternately, if that light is focused
on a block of charcoal, a great deal of the light will be absorbed into
the charcoal and little will be reflected back. The light absorbed by
the charcoal will perform "work" in that the charcoal will become
heated. This may be a clumsy example, but keep in mind the concept of
doing "work". A similar concept is the selection of proper gear ratio
when riding a bicycle up a hill. The proper ratio (match) yields the
best "work".

Ideally if a device such as a balun is properly constructed and
terminated properly, the amount of power reflected will be zero.
Unfortunately this is seldom the case and some amount of power will be
reflected back. The ratio of the power into the device and the power
reflected back is the return loss which is often expressed in dB's.
Another figure of merit is voltage standing wave ratio (VSWR). The
following table depicts the relationship between VSWR and return loss. A
device with a return loss of 20 dB is better than a device with a return
loss of 14 dB.


http://www.minicircuits.com/pages/pdfs/dg03-111.pdf


As far as your question about the passive devices between the chip and
the antenna, the easy answer is "yes" however, proper matching of the
device requires some passive components, so it would be safe to assume
if the manufacturer recommends a certain matching network to yield a
specific receiver sensitivity threshold, those components used in an
evaluation circuit would already be included in the terminal sensitivity
figure. Usually manufacturers will recommend a specific network be used
to measure parameters of an RF device in their application notes.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

 

[Michael]

Insertion loss is expressed in dB and pertains to the efficiency of the
balun when passing a signal. A balun with a 1.2 dB insertion loss is
somewhat less efficient than one with 0.2 dB IL. For example if a device
has a 3 dB insertion loss and you generate 100 watts (+50 dBm) at its
input you will lose 1/2 the power and will have only 50 watts (+47 dBm)
at the output. Additionally the device has to dissipate the lost energy
as heat and this will also affect its longevity, unless it is power
rated accordingly.

The insertion loss specification assumes that the device is properly
terminated, bringing us to the next question:

This makes perfect sense. Just to be sure, however: the insertion loss
goes both ways, right? In other words - both received and transmitted
signals suffer from insertion loss - so a 1dB insertion loss really
means a 2dB link loss as it'll happen on both the receiving and
transmitting ends of the link.

Return loss pertains to the power reflected back from a device at its
input (and likewise output) port. Return loss is measured with a return
loss bridge or other device. This is a figure of merit of the proper
matching of the source impedance to the load impedance.

Imagine a spot light focused on a mirror, nearly all of the power will
be reflected back to the source. Alternately, if that light is focused
on a block of charcoal, a great deal of the light will be absorbed into
the charcoal and little will be reflected back. The light absorbed by
the charcoal will perform "work" in that the charcoal will become
heated. This may be a clumsy example, but keep in mind the concept of
doing "work". A similar concept is the selection of proper gear ratio
when riding a bicycle up a hill. The proper ratio (match) yields the
best "work".

Ideally if a device such as a balun is properly constructed and
terminated properly, the amount of power reflected will be zero.
Unfortunately this is seldom the case and some amount of power will be
reflected back. The ratio of the power into the device and the power
reflected back is the return loss which is often expressed in dB's.
Another figure of merit is voltage standing wave ratio (VSWR). The
following table depicts the relationship between VSWR and return loss. A
device with a return loss of 20 dB is better than a device with a return
loss of 14 dB.

http://www.minicircuits.com/pages/pdfs/dg03-111.pdf

As far as your question about the passive devices between the chip and
the antenna, the easy answer is "yes" however, proper matching of the
device requires some passive components, so it would be safe to assume
if the manufacturer recommends a certain matching network to yield a
specific receiver sensitivity threshold, those components used in an
evaluation circuit would already be included in the terminal sensitivity
figure. Usually manufacturers will recommend a specific network be used
to measure parameters of an RF device in their application notes.

So on page 152 of the AT86RF212 datasheet (http://www.atmel.com/dyn/
resources/prod_documents/doc8168.pdf) it lists rx sensitivity at -92dB
for 1Mbps and nominal output power at 5dBm. You think these figures
are taking into account the balun and other passives shown in the
example circuit (page 10).

Thanks!

-Michael

 

[RFI-EMI-GUY]

See my answers embedded below:

This makes perfect sense. Just to be sure, however: the insertion loss
goes both ways, right? In other words - both received and transmitted
signals suffer from insertion loss - so a 1dB insertion loss really
means a 2dB link loss as it'll happen on both the receiving and
transmitting ends of the link.

Yes this is typical for passive devices. There is at least one
exception- RF Isolators (AKA Circulators) that have an insertion loss in
the forward direction and an isolation loss in the reverse direction. Of
course a passive has to be in both the transmit and receive path to
affect both equally. For example a filter used to clean up the
transmitter can be put between the antenna and the antenna switch to
both clean up the transmitter and provide frequency selectivity
protection to the receiver, but doing so, will reduce the path
performance by 2X its insertion loss (once at each end of the link). If
this component does not truly provide a benefit to the receiver, it
should be placed between the transmitter and the antenna switch.

So on page 152 of the AT86RF212 datasheet (http://www.atmel.com/dyn/
resources/prod_documents/doc8168.pdf) it lists rx sensitivity at -92dB
for 1Mbps and nominal output power at 5dBm. You think these figures
are taking into account the balun and other passives shown in the
example circuit (page 10).

I would expect so. If you build the circuit and you complain to the
manufacturer that you don't measure the RX sensitivity or power output,
the manufacturer is going to ask if you built it to the application note
specs using the same parts. I would however caution that filter 1 (F1)
is optional and the vendor may be "forgetting" to include that as a
worst case. Also beware of words like "typical", may mean that they are
not guaranteeing the performance. I am sure you are aware that the data
rate, modulation scheme, transmitter masks and receiver IF filters will
all affect the link margin.

You should build a link, put each transceiver in a well shielded can and
use a variable attenuator to simulate the link performance. Beware that
step attenuators may introduce glitches that will cause the link to drop
out. If you can obtain a truly variable attenuator as part of your test
set up, you can approach the threshold more smoothly.

Since we are on the discussion about link budgets, I suggest you read up
about log normal fading, Rayleigh fading etc, so that you don't rely
entirely on free space loss and become disappointed in the real world
performance. There are more documents than I could even list here that
cover the subject of RF propagation. Fortunately there is a lot of study
in that area.

See my answers embedded above:

Thanks!

-Michael

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P