EMRFD Message Archive 703

Message Date From Subject
703 2007-05-03 16:19:08 ka3j12 VFO level for SA612 Mixer
Hi - I understand that 700 mV pk-pk is about what the SA612 likes to
see when used as a typical first mixer in a superhet. Has anyone
experimented, analyzed or care to speculate regarding the impact of
running with a lower VFO signal level? Thanks!


Ron (KA3J)
704 2007-05-03 16:51:03 Luiz Amaral Re: VFO level for SA612 Mixer
This is said to be the condition for best conversion gain and best 3rd IMD

Luiz - PY1LL / PY4LC

----- Original Message -----
706 2007-05-03 17:23:59 DSNman@comcast.ne... Re: VFO level for SA612 Mixer
Attachments :

    I would guess it would cause lower conversion gain and maybe more easily overloaded by strong signals.

    Loren Moline

    -------------- Original message ----------------------
    From: "ka3j12" <rsstone@juno.com>
    > Hi - I understand that 700 mV pk-pk is about what the SA612 likes to
    > see when used as a typical first mixer in a superhet. Has anyone
    > experimented, analyzed or care to speculate regarding the impact of
    > running with a lower VFO signal level? Thanks!
    > 73,
    > Ron (KA3J)
    707 2007-05-03 19:12:08 Kevin Purcell Re: VFO level for SA612 Mixer
    From the datasheet for the 612

    > External L.O. should be 200mVP-P minimum to 300mVP-P maximum.

    so 700mV is pretty hard drive though they do drive the 602 with 630mV
    on EMRFD page 5.10 which is a bit high. Curiously they get an IIP3
    of -17.5 dBm which is rather worse than the typical -13dBm (though
    their measured gain is also 3dB up which might explain it).

    With the 602/612 you don't drive the "top" transistors directly there
    is the oscillator transistor, biased for about 0.12mA emitter
    current, and an unspecified buffer in the way. But I'm pretty sure
    they're in the hard switching regime. It might be that given the
    design of the buffer amps it's difficult to get to the linear regime
    (if the buffer is not very linear).

    You might to read the 1496 app note that I posted a link to a few
    days ago as I think it may be relevant here.

    Both mixers are gilbert cell designs but the 1496 can be run in
    either "switching mode" with high level drive or a "linear mode" low
    level drive with the crossover point being 15 to 20mV RMS or 40 to
    60mV pkpk (though this varies depending on the gain of the device and
    the standing current in the diff pair).

    For the 1496 the voltage gain drops to about half in the "low drive"
    case but so does the number of spurious outputs (no more odd
    harmonics in the LO). The drop in gain should improve the input IP3
    and so the spurious free dynamic range (in the 1496 at least). The
    noise figure also increases as the gain drops but for HF that might
    not be a bad point. Also amplitude variations in the LO will appear
    in the output too (but you should be able to mitigate that).

    The gain also drops as you increase the diff pair standing current
    but NF improves.

    The original Gilbert patent 3689752


    and this one is interesting because it has an expression for the IMD
    and how it depends on the emitter current (more is better), emitter
    resistor (larger is better) and the input impedance (lower is better).


    IP3 = 10 + 20 log(104) + 30 log (1 + (R_E I_EE)/104) - 10 log (R_i/50)

    R_E is the emitter degeneration resistor in kohms and I_EE is the
    emitter current in mA. R_i is the input impedance in ohms.

    By inspection the IP3 goes as the cube of R_E and the cube of I_EE.
    So turning up the current has a large effect on IP3. Likewise turning
    down the gain has a strong effect on IP3.

    For R_E=1kohm; I_EE=1mA; R_i=1.5kohm the input IP3 is -15dBm. Seems
    about right.

    For R_E=1kohm; I_EE=10mA; R_i=1.5kohm the input IP3 is +15dBm. Seems
    good. Perhaps too good. I'm pretty sure these parameters interact
    with others :-)

    The relates to some earlier comments on 1496 thread by Allison.

    I'm still writing a response to that but the idea is it would be
    interesting to see the 1496 characterized for high standing current
    (~10mA max), low gain and low LO drive (rather than from it's normal
    parameters -- 1mA standing current and high level drive) and see
    what effect that has on the intercept and noise figure. The part is
    also designed to run at up to 30V. I wonder what effect increasing
    the Vcc has on IP3?

    Fo example the discrete Gilbert Cell mixer on p5.11 was biased at 5mA
    and 15mA and gives much better numbers (as you might expect) with an
    SSB noise figure of 13.8dB.

    15mA standing current, gain 18dB, IIP3 +11dBm with 0dBm LO drive
    5mA standing current, gain 16dB, IIP3 -2dBm with 0dBm LO drive

    So given that and the other expression presented above it seems like
    it might be worthwhile to characterize these devices when "pushed".
    They're not going to exceed a diode ring mixer but they might still
    give decent results (and be easier to get hold of in VU-land).

    The other cryptic comment (one of the many!) in EMRFD is:

    > Although the main tool used to improve IMD performance in a Gilbert
    > Cell is to increase current, feedback can also be applied. The
    > experimenter should examine the work of Trask.

    And that's Chris Trask, N7ZWY, but unfortunately it's not an online
    paper but some of his other active mixers with lossless feedback
    papers are.


    See also

    After all EMRFD is about experimentation, right.


    709 2007-05-04 11:02:23 Dave Re: VFO level for SA612 Mixer
    712 2007-05-04 13:33:54 Kevin Purcell Re: VFO level for SA612 Mixer
    Did you measure any other parameters?


    noise figure?

    717 2007-05-04 18:50:46 Kevin Purcell Gilbert Cell mixers with good performance
    Of Chris Trask's papers this one is perhaps the most relevant to this
    discussion -- comparing a linearized Gilbert cell mixer with feedback
    to a diode ring mixer. It's nice overview of trade offs with
    different mixer designs too. And he reports measured numbers!


    Notice the "regular" Gilbert cell mixer parameters with small emitter
    degeneration resistor and biasing set to keep the lower transistor
    linear at all times. With a 15mA standing current and 0dBm LO drive
    he measures

    IIP3 +17.5dB
    Gain -1.5dB
    P 1dB +4.5dB

    No mention of the noise figure.

    He says:

    > It has long been recognized that the most
    > serious limitation in the IMD performance of tree
    > mixers is that of the voltage-to-current conversion
    > of the driver transistors Q3 and Q6 Various
    > methods have been utilized sucessfully to correct
    > this deficiency but these methods all ignore
    > secondary sources of intermodulation, primarily the
    > hfe nonlinearity of the driver transistors and the
    > nonlinear charcteristics of the four switching
    > transistors. These deficiencies can be overcome
    > by making use of a simple series/shunt feedback
    > amplifier circuit wherein all of the transistors are
    > embedded within the feedback topology.

    and then goes on to show his series/shunt feedback amplifier
    modifications. The first version just adds four resistors from the
    collectors at the top of the mixer tree to the corresponding bases on
    the "same side" of the tree.

    With a 15mA standing current and 0dBm LO drive he measures

    IIP3 +21.5dB
    Gain -7dB
    P 1dB +5.5dB

    This technique might work with the 1496 too. You can get to all the
    terminals of the transistors to add the feedback resistors (a win
    over the 602 for an experimenter) but you can't control the emitter
    resistors at the bottom of the tree (though you can control the bias
    on the constant current transistors. If I'm wrong on any of this let
    me know :-)

    The Gilbert cell mixers also have an advantage in being a lot less
    sensitive to IF port termination unlike commutative mixers. Below the
    comparison is between a perfect termination (Without IF Filter) and
    into a resonant bandpass filter (With IF Filter).

    Mini-Circuits SBL-1
    Without IF Filter P 1dB −4.5dBm IIP3 +19.0dBm
    With IF Filter P 1dB −7.5dBm IIP3 +7.5dBm

    Transistor Tree Mixer (aka Gilbert Cell)
    Without IF Filter P 1dB +4.5dBm IIP3 +17.5dBm
    With IF Filter P 1dB +4.5dBm IIP3 +16.5dBm

    Linearized Feedback Mixer
    Without IF Filter P 1dB +5.5dBm IIP3 +21.5dBm
    With IF Filter P 1dB +5.5dBm IIP3 +20.75dBm

    I'm also reminded (being a G and avid reader of Technical Topics in
    my youth) of the, now obsolete, Plessy SL6440. It's use was
    restricted to mostly British and Commonwealth (G, and some VK, ZL,
    ZS) RF designs though it appeared in a lot of commercial and military
    radios. It's interesting how parochialism (and availability of
    products) constrain designs even when an excellent part is available.

    But you can at least still get the datasheet


    It was good for HF and low VHF use (3dB down at 100MHz min spec) with
    very good numbers with a 250mV rms LO drive (into 1.5kohms)

    IIP3 +30dBm (with a standing current of 50mA)
    Gain -1dB
    P 1dB +15dBm
    NF 11dB

    Not bad for a Gilbert Cell mixer and even the NF was OKish :-)

    A Plessey application note AN-156-2 "Intermodulation, Phase Noise and
    Dynamic Range" explains why Gilbert cell mixers have these issues.
    it's also interesting for linking phase noise to dynamic range
    consideration (and has a fun appendix on where IMD arises and how to
    calculate it).


    > Many advantages accrue to the choice of the transistor tree
    > type of approach. Here the input signal produces a
    > current in the collectors of the lower transistors and this current
    > is commutated by the upper set of switching transistors.
    > Because the current is to a first order approximation independent
    > of collector voltage, the transistor tree does not exhibit the
    > sensitivity to load impedance that the diode ring does, and
    > indeed, by the use of suitable load impedances, gain may be
    > achieved. The non- linearity of the voltage to current conversion
    > in the base emitter junctions of the bottom transistors is the
    > major cause of intermodulation, but by using suitably large
    > transistors and
    > emitter degeneration, very high performances ( +32dBm input
    > intercept) can be achieved.

    The SL6440 was an improvement on the standard Gilbert cell mixer
    design ...

    > In an attempt to improve this trade-off, P. E. Chadwick, "The
    > SL6440 High Performance Integrated Circuit Mixer", Plessey
    > Semiconductors Limited, 1981, discloses that complementary
    > linearising transistors can be added to the lower pair of
    > transistors to provide negative feedback. However, this approach
    > has the disadvantages of introducing noise and presenting bias
    > stability problems for practical ranges of operating temperature
    > and integrated circuit process parameters.

    That's Peter Chadwick, G3RZP.

    BTW he has this to say about low LO drive into a Gilbert cell mixer


    > Bill said:
    >> But if the local oscillator signal is strong enough to simply
    >> turn on
    >> and off the mixer tube it does become a very effective >frequency
    >> mixer.
    >> It then appears to be an analog mixer multiplying a signal with a
    >> square
    >> wave.
    > It does reduce the conversion gain, though. That's the big problem
    > with
    > switching mixers. The so called 'Gilbert cell' (which Barrie Gilbert
    > doesn't claim as his - it was first patented by a guy from RCA as a 4
    > quadrant multiplier - Barrie's claim is that he was the first to
    > suggest
    > using it as a mixer, and even that's disputed) can be oeprated in
    > lower
    > gain as a switching mixer, and in higher gain as an analogue
    > multiplier
    > type mixer, with lower LO drive and worse IMD performance. It also
    > is more
    > susceptible to AM on the LO.
    > 73
    > Peter G3RZP.

    So my previous suggestion of trying lower drive and operating the
    1496 as a linear multiplying mixer is not a good idea.

    So how to compare mixers. That's the next email.

    768 2007-05-14 20:52:37 Kevin Purcell Re: VFO level for SA612 Mixer
    Whilst hunting around for other information I came across Cosy MUTO,
    JH5ESM, who has measured this for a 602 + NJM4580 op amp.


    > Gain is proportional to LO levels under -20[dBV] (0.1[Vrms])..
    He gives gains in dB but as they're not between matched impedances
    they're not really power gains just "voltage gains converted to dB".

    > When the gain is proportional to LO level (i.e., under -20[dBV]),
    > AM feedthrough seems to be constant. However, it goes wrong by 10
    > [dB] when the mixer is overdriven.
    > Therefore, LO level should be carefully chosen to obtain best
    > results, that is -20[dBV] in this case.
    He concluded for a DC RX front end

    > Optimum LO level is -20[dBV] (=0.1[Vrms]) under 5.0[V] supply voltage
    Which is about 280mV pk-pk.


    777 2007-05-20 13:45:11 Dave Re: VFO level for SA612 Mixer

    Sorry for the delayed reply...I don't check the Yahoogroups very
    often. No, I did no other measurements