[minima] Re: RX gain of Bidirectional Amp?
- From: Dale Ward <adwsail@xxxxxxxxx>
- To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
- Date: Thu, 16 Oct 2014 20:53:13 -0400
Thanks guys, time to start building.
Dale
WB4LIP
On Thursday, October 16, 2014, Steve VK2SJA <vk2sja@xxxxxxxxxxx> wrote:
>
>
> >Is there a link somewhere to your construction guide?
>
> minima_main_page | ae5phradionotebook
> <https://sites.google.com/site/ae5phradionotebook/minima_main_page>
>
> Down towards the bottom of the page.
>
>
>
> >
> > WB4LIP
> >
> > On Thu, Oct 16, 2014 at 12:10 AM, Mac A. Cody <maccody@xxxxxxx
> <javascript:;>> wrote:
> >
> >> Steve,
> >>
> >> Sorry about the misunderstanding. I had forgotten that the
> >> circuitry of the bidirectional amp was not symmetrical.
> >>
> >> Glad you like my construction guide. I've taken a bit of
> >> a 'breather' from working on it due to time, other projects,
> >> waiting for design issues to sort out, and a bit of personal
> >> 'burnout' on the Minima project. The notes that you are
> >> working on is a great idea.
> >>
> >> 73,
> >>
> >> Mac AE5PH
> >>
> >>
> >> On 10/14/2014 10:03 PM, Steve VK2SJA wrote:
> >>
> >>> Thanks Mac,
> >>>
> >>> This is Joe's own suggestion for the TX side of the bidirectional amp.
> >>> Joe
> >>> and I were talking about the other side/direction, the RX side of the
> >>> amp.
> >>>
> >>> I think Joe was saying he thought that he may have seen somebody else's
> >>> suggested mod to reduce current here? At least that's how I was reading
> >>> it.
> >>>
> >>> Great "Construction Guide" BTW.
> >>>
> >>> 73, Steve.
> >>>
> >>>
> >>>
> >>> Steve,
> >>>>
> >>>> I've copied Joe Rocci's original post below.
> >>>>
> >>>> TL'DR:
> >>>> Change the 220 Ohm collector resistor to 1000 Ohms to decrease
> >>>> the current consumption. Increase the emitter degeneration
> >>>> resistor to 20 Ohms to bump the gain up to 20 dB.
> >>>>
> >>>> 73,
> >>>>
> >>>> Mac AE5PH
> >>>>
> >>>> Original message posted by Joe Rocci:
> >>>> Hi Steve
> >>>>
> >>>> Here are some results from modeling the Tx section in LTSpice:
> >>>>
> >>>> 1) Gain is about 17db @ 20 mhz
> >>>> 2) Input match (return loss) is about 16db and pretty flat up to 50
> >>>> Mhz
> >>>> 3) Output match is quite good, at the expense of about 6db gain loss,
> >>>> due to the series output matching resistor
> >>>> 4) Current draw is about 55ma
> >>>>
> >>>> Some comments:
> >>>> 1) The voltage-amp stage is biased at almost 11 volts on the
> >>>> collector,
> >>>> leaving very little headroom for large voltage swings.
> >>>> 2) The emitter follower buffer stages draw the lion's share of the DC
> >>>> current, but are non-optimum for driving large voltage swings into
> >>>> non-resistive loads at high frequencies; that's why so much static DC
> >>>> current is required.
> >>>> 3) If you increase the 220 ohm collector resistor to 1000 ohms, the
> >>>> collector drops closer to 5.5 V, and the overall current consumption
> >>>> drops to about 28 ma. If you then increase the emitter degeneration
> >>>> resistor to 20 ohms, you end up with about 20 dB gain (@20 Mhz), 22 dB
> >>>> input match, and presumably much better output voltage swing headroom.
> >>>> I
> >>>> discovered all this while trying to goose the amp up for a little
> >>>> better
> >>>> performance at 30 Mhz, anticipating a new crystal filter.
> >>>>
> >>>> Of course, all of this is 'on paper' and needs to be verified.
> >>>>
> >>>> Joe
> >>>> W3JDR
> >>>>
> >>>>
> >>>> On 10/14/2014 08:22 PM, Steve VK2SJA wrote:
> >>>>
> >>>>> Thanks Joe,
> >>>>>
> >>>>> So ~16 to 20dB in both directions.
> >>>>>
> >>>>> I had a quick look around the archive this morning. If there is a mod
> >>>>> for
> >>>>> reducing current in the RX side of the bidirectional amp I've managed
> >>>>> to
> >>>>> miss it.
> >>>>>
> >>>>> Anyone know of such a mod?
> >>>>>
> >>>>> The original W7ZOI bidirectional amp article quotes a noise figure of
> >>>>> 5.8dB when the amp was setup for about 16dB of gain. So 4.5dB seems
> >>>>> an
> >>>>> improvement on that and around the same ball park. So the LTSpice
> >>>>> simulation is probably on the money.
> >>>>>
> >>>>> Last night I was reading EMRFD and learnt how to go about measuring
> >>>>> noise
> >>>>> properly. I've ordered a LNA and when it arrives I'll try measure it
> >>>>> in
> >>>>> the real-world and report back.
> >>>>>
> >>>>> I've been reading that at HF frequencies there is so much atmospheric
> >>>>> noise that a 3kHz wide SSB rig will typically see about 14dB of
> >>>>> noise.
> >>>>> And
> >>>>> that as such any amp with a noise factor (NF) of of a bit less than
> >>>>> ~14dB
> >>>>> is probably adequate for this application. Because you won't see the
> >>>>> locally generated amp noise over the atmospheric noise anyway. So
> >>>>> while
> >>>>> we
> >>>>> should always design and build as best we can. From a practical stand
> >>>>> point is there anything to be gained by reducing the NF to a figure
> >>>>> much
> >>>>> below ~6dB?
> >>>>>
> >>>>> 73, Steve.
> >>>>>
> >>>>>
> >>>>>
> >>>>> Steve
> >>>>>>
> >>>>>> LTSpice simulation indicates about 16db gain at 20 mhz with the
> >>>>>> component
> >>>>>> values shown on the web page. The standing current is also very
> >>>>>> high,
> >>>>>> at
> >>>>>> over 40ma. I think there was a mod to bring the current down? Also,
> >>>>>> the
> >>>>>> simulated noise figure is about 4.5db, pretty high. However, I've
> >>>>>> never
> >>>>>> checked to see how accurate LTSpice is in this regard.
> >>>>>>
> >>>>>> Joe
> >>>>>>
> >>>>>> ----- Original Message -----
> >>>>>> From: "Steve VK2SJA" <vk2sja@xxxxxxxxxxx <javascript:;>>
> >>>>>> To: <minima@xxxxxxxxxxxxx <javascript:;>>
> >>>>>> Sent: Tuesday, October 14, 2014 6:40 AM
> >>>>>> Subject: [minima] RX gain of Bidirectional Amp?
> >>>>>>
> >>>>>>
> >>>>>> Hi Guys,
> >>>>>>>
> >>>>>>> The gain of the Minima Bidirectional Amp in the TX direction is ~16
> >>>>>>> to
> >>>>>>> 20dB.
> >>>>>>>
> >>>>>>> Can someone tell me what the gain is supposed to be for the other
> >>>>>>> half
> >>>>>>> of
> >>>>>>> the duel Amp, in the RX direction? Same?
> >>>>>>>
> >>>>>>> 73, Steve.
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>
> >>>>>
> >>>>>
> >>>>
> >>>
> >>>
> >>>
> >>
> >>
> >
>
>
>
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